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Environmental Protection 
Agency 


Office of Research and 
Development 
Washington DC 20460 


EPA/600/R-98/058 
August 1998 


SERA Application of the 

Electromagnetic Borehole 
Flowmeter 














EPA/600/R-98/058 
August 1998 


Application of the 
Electromagnetic 
Borehole Flowmeter 


by 


Steven C. Young, Hank E. Julian, and Hubert S. Pearson 
Tennessee Valley Authority 
Engineering Laboratory 
Norris, TN 37828 


and 


Fred J. Molz and Gerald K. Boman 
Auburn University 
Auburn, AL 36849 


Interagency Agreement 
DW64934812 


Project Officer 
Steven D. Acree 

Subsurface Protection and Remediation Division 
National Risk Management Research Laboratory 
Ada, OK 74820 


National Risk Management Research Laboratory 
Office of Research and Development 
U.S. Environmental Protection Agency 
Cincinnati, OH 45268 


Printed on Recycled Paper 


Notice 


The U.S. Environmental Protection Agency, through its Office of Research and Development, partially funded and collaborated 
in the research described here under Interagency Agreement DW64934812 to the Tennessee Valley Authority. It has been subjected 
to the Agency’s peer and administrative review and has been approved for publication as an EPA document. Mention of trade names 
or commercial products does not constitute endorsement or recommendation for use. 

All research projects funded by the U.S. Environmental Protection Agency that make conclusions or recommendations based on 
environmentally related measurements are required to participate in the Agency Quality Assurance Program. This project was 
conducted under an approved Quality Assurance Project Plan and the procedures therein specified were used. Information on the 
plan and documentation of the quality assurance activities and results are available from the Principal Investigator. 


/ C /?/ 
•Ac/ 

) m 




11 



Foreword 


The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation’s land, air, and water resources. 
Under a mandate of national environmental laws, the Agency strives to formulate and implement actions leading to a compatible 
balance between human activities and the ability of natural systems to support and nurture life. To meet these mandates, EPA’s 
research program is providing data and technical support for solving environmental problems today and building a science knowledge 
base necessary to manage our ecological resources wisely, understand how pollutants affect our health, and prevent or reduce 
environmental risks in the future. 

The National Risk Management Research Laboratory is the Agency’s center for investigation of technological and management 
approaches for reducing risks from threats to human health and the environment. The focus of the Laboratory’s research program is 
on methods for the prevention and control of pollution to air, land, water, and subsurface resources; protection of water quality in 
public water systems; remediation of contaminated sites and ground water; and prevention and control of indoor air pollution. The 
goal of this research effort is to catalyze development and implementation of innovative, cost-effective environmental technologies; 
develop scientific and engineering information needed by EPA to support regulatory and policy decisions; and provide technical 
support and information transfer to ensure effective implementation of environmental regulations and strategies. 

This report presents a discussion of the applications of vertical-component borehole flowmeters to site characterization with an 
emphasis on the design and use of a sensitive electromagnetic flowmeter partially developed under this project. The methodologies 
discussed in this document provide cost-effective means for obtaining detailed definitions of hydrogeologic controls on ground- 
water flow and contaminant transport. Such information is often essential for evaluation of contaminant fate in the environment and 
design of effective monitoring and remediation systems. It is published and made available by EPA’s Office of Research and 
Development to assist the user community. 



Subsurface Protection and Remediation Division 
National Risk Management Research Laboratory 


111 



Abstract 


A prerequisite for useful monitoring, modeling, and remedial design strategies is knowledge of the network of hydraulically 
active fractures in bedrock aquifers and three-dimensional hydraulic conductivity fields in granular aquifers. Due to a relative lack 
of practicable characterization technologies, many ground-water remediation strategies have been designed based on very little, if 
any, detailed information regarding fracture or hydraulic conductivity distribution. As a result, the underestimation of aquifer 
heterogeneity may contribute to inadequate conceptual models of contaminant transport/fate and inadequate design/performance of 
many remediation systems. 

Borehole flowmeters are effective tools for measuring vertical variations in the flow field of an aquifer. Although borehole 
flowmeters have been used in the petroleum industry for decades, they are not common in ground-water studies due, in part, to the 
lack of suitable meters. A vertical component electromagnetic (EM) borehole flowmeter with the versatility and sensitivity required 
for application at most sites has been developed by the Tennessee Valley Authority (TVA). The prototype TVA EM flowmeters have 
outer diameters of less than 5.25 cm, and inner diameters of either 2.54 cm or 1.27 cm. The detection limits for the 1.27-cm and 2.54- 
cm flowmeters are near 0.005 L/min and 0.03 L/min, respectively. In addition to a low detection limit, attractive features of the 
prototype and commercial versions of this EM flowmeter include a wide measurement range, durable construction, no moving parts, 
and a design that facilitates use with packer assemblies. 

This report describes the operation and application of the TVA prototype EM borehole flowmeters, including theory, design, 
calibration, basic field applications, data analysis, and potential effects of various well construction and development procedures on 
data. The majority of these results are also applicable to the commercial version of this meter and other vertical component borehole 
flowmeters, including heat pulse and impeller tools. Several case studies illustrating specific uses of these tools are also discussed. 



Contents 


Section Page 


Notice.ii 

Foreword.iii 

Abstract.iv 

List of Figures.vii 

List of Tables..<.ix 

I. Theory and Application of the Electromagnetic Borehole Flowmeter.1 

1-1. Overview.1 

1-2. Principles of Operation.2 

1-3. Collection of Flowmeter Logs.2 

1-4. Analysis of Flowmeter Logs.2 

a. Flow Distribution Logs. 2 

b. Hydraulic Conductivity Logs in Granular Aquifers.6 

c. Data Analysis. 7 

I- 5. Conclusions. 8 

II. Electromagnetic Borehole Flowmeter System. r .9 

II- 1. Design and Construction.9 

II- 2. Laboratory Calibration. 10 

III. Hydrogeologic Characterization Test Design. 13 

III- 1. Overview.:.13 

III-2. Measurement Interval Selection.13 

III-3. Flowmeter Measurements. 13 

III-4. Pumping Rate and Drawdown Measurements.14 

III-5. Selection of Pumps.14 

III-6. Packer Selection.14 

III-7. Method of Inflation ..... v ..15 

III-8. Issues Regarding Flowmeter Measurements in the Field..15 

a. Well or Borehole Storage.15 

b. Selection of Pumping Rate.15 

c. Background Electromagnetic Currents.16 

d. Reproducibility of Flowmeter Data. 16 

III- 9. Flowmeter Investigations in Consolidated Materials.16 

a. Introduction. 16 

b. Fractured Rock Applications.17 

c. Conclusions.18 

IV. Well Construction and Development.19 

IV- 1. Background.19 

a. In-Situ Hydraulic Conductivity Estimates Using Wells.19 

b. Formation Damages and Skin Effects.19 

IV-2. Well Design.19 

IV-3. Well Installation Methods. 20 

a. Overview.-..20 

b. Drilling Techniques and Hydraulic Conductivity Estimates.20 


v 
















































Contents 

(continued) 

Section Page 


IV- 4. Well Development Methods.21 

a. Overview.21 

b. Development Methods and Hydraulic Conductivity Estimates.21 

V. Field Studies of Well Construction and Development at Columbus, Mississippi.23 

V- 1. Description of Test Site.23 

a. Site Location.23 

b. Aquifer Characteristics.23 

c. Previous Pumping Tests.23 

d. Monitoring Well Installation.24 

V-2. Test Descriptions.25 

V-3. Test Analyses and Results.26 

a. Ambient Flow Distributions.26 

b. Induced Flow Distributions.28 

c. Specific Capacity Values.28 

d. Transmissivity Values.28 

V- 4. Summary.31 

V 

VI. Field Studies of Well Construction and Development at Mobile, Alabama.,.33 

VI- 1. Description of Test Site.33 

a. Site Location.33 

b. Aquifer Characteristics.33 

c. Well Installation.33 

VI-2. Test Description.34 

VI-3. Test Results.35 

a. Shallow Well A.35 

b. Shallow Well B.36 

c. Deep Well C.38 

VI- 4. Summary and Discussion.40 

VII. Case Studies. 43 

VII- 1. Field Applications.43 

VII-2. Columbus AFB, Mississippi.43 

VII-3. Oak Ridge National Laboratory, Tennessee.45 

VII-4. The Oklahoma Refining Company Superfund Site, Oklahoma.46 

VII-5. Gilson Road Superfund Site, New Hampshire.46 

VII-6. Mirror Lake, New Hampshire.46 

VII-7. Logan Martin Dam, Alabama.48 

VII-8. Cape Cod, Massachusetts.50 

VII-9. Summary.51 

VIII. References.53 

APPENDIX A. Field Data Sheets for Borehole Flowmeter Tests.57 

APPENDIX B. Equipment Checklist.59 


vi 













































Figures 


Number Page 

1-1 Schematic diagram of the TVA electromagnetic borehole flowmeter.3 

1-2 Apparatus and geometry associated with a borehole flowmeter test.3 

1-3 Graphical illustration of the hypothetical case presented in Table 1-1.4 

I- 4 Assumed layered geometry within which flowmeter data are collected and analyzed.5 

II- 1 Mechanical packer (a) assembled on probe and (b) in schematic view.9 

II-2 Inflatable packer assembled on probe.10 

II-3 Conceptual system for flowmeter calibration.10 

II-4 Calibration data for the 1.27-cm ID flowmeter in 5.1 -cm PVC pipe. 11 

II- 5 Frictional losses associated with the 1.27-cm and 2.54-cm ID flowmeters.12 

III- 1 Acoustic-televiewer, caliper, single-point resistance, and flowmeter logs for borehole 

DH-14 in northeastern Illinois (Paillet and Keys, 1984; Molz et al., 1990). 17 

V-l Ox bow meander at the Columbus AFB site drawn from a 1956 aerial photograph.23 

V-2 Well network at the 1-Ha test site.24 

V-3 Design of wells used to evaluate the effect of well development on flowmeter tests.25 

V-4 Ambient flow profiles after successive well development.27 

V-5 Induced flow profiles after successive well development.29 

V-6 Effect of well development on the drawdown values for pumping tests at the 38-39-40 well cluster.30 

V-7 Comparison of specific capacities at different wells.31 

V- 8 Calculated transmissivities using early time data.32 

VI- 1 Vertical cross-sectional illustration of the subsurface hydrologic system at the Mobile site.33 

VI-2 Schematic diagram providing the details of the shallow and deep wells constructed at the Mobile site. 

Well casing and well screen are the same dimension and schedule. The only difference is the depth 

dimensions of the casing and screen.34 

VI-3 (Well A) (a) Ambient flow as measured by the EM flowmeter and (b) net flow (total flow - 

ambient flow). Data were obtained after the 2nd and 3rd developments.36 

VI-4 (Well A) Differential net flow obtained (a) after 2nd development, (b) after 2nd 

development and an overnight waiting period, and (c) after 3rd development.37 

vii 



























Figures 

(continued) 


Number Page 

VI-5 (Well B) (a) Ambient flow as measured by the EM flowmeter and (b) net flow (total flow - ambient flow). 

Data sets were obtained prior to development and after 1st, 2nd, and 3rd developments.38 

VI-6 (Well B) Differential net flow obtained (a) prior to development, (b) after 1st development, (c) after 2nd 

development, and (d) after 3rd development.39 

VI-7 (Well B) Ambient flow after the first development and repeated after an overnight period.40 

VI-8 (Well C) (a) Ambient flow as measured by the EM flowmeter and (b) net flow (total flow - 
ambient flow). Data were obtained prior to development and after 1st, 2nd, and 

3rd developments for the ambient flow and after 1st, 2nd, and 3rd developments for net flow.40 

VI- 9 (Well C) Differential net flow obtained (a) after 1st development, (b) after 1st development and overnight 

waiting period, (c) after 2nd development, and (d) after 3rd development.41 

VII- 1 Vertical profile of hydraulic conductivity values along the longitudinal axis of the MADE tracer plume.44 

VII-2 Depth-averaged hydraulic conductivity values for the lowermost 2 m (left) and the uppermost 2 m (right) 

of the unconfined aquifer below the 1-Ha test site at Columbus AFB .44 

VII-3 (a) Ambient flow distribution in a well and (b) the flow distribution caused by constant-rate pumping.45 

VII-4 Ambient and induced flow distributions for wells NE-2, SBB-36, and IBB-4 at the ORC Superfund Site.47 

VII-5 Ambient and induced flow distributions for wells I, J, and K at the Gilson Road site.47 

VII-6 Ambient and induced flow distributions for wells FSE-06, FSE-09, and FSE-10 at Mirror Lake, 

New Hampshire.49 

VII-7 Substantial ambient flow moving from one stratum to another as detected by an impeller flowmeter. The 
flow is moving under a dam, the base of which is at an elevation of about 140 m AMSL. Flowmeter 
data were used to help select a geologic model for explaining the large amount of leakage of water low 
in dissolved oxygen that was observed below the dam.50 


viii 

















Tables 


Number Page 

1-1 Hypothetical data and data analysis related to the application of a borehole flowmeter.6 

I- 2 Results of the analysis of data from a borehole flowmeter test to estimate hydraulic conductivity distributions.8 

II- 1 Sample calibration factors (liter/min/volt) from a linear regression of discharge versus voltage for 

electromagnetic flowmeter data.12 

III- 1 Generalized borehole flowmeter field test procedures.13 

III-2 Problems that produce errors in EM flowmeter measurements.14 

V-l Testing sequence.26 

V- 2 Early-time CJSL transmissivity values.32 

\ 

VI- 1 Pump-induced flow and ambient flow tests performed on wells A, B, and C.35 

VII- 1 Characterization objectives for borehole flowmeter studies.43 

VII-2 Summary statistics of hydraulic conductivity data obtained using a borehole flowmeter method 

and a permeameter analysis method (Hess et al. (1992)).51 


IX 




















































. 











































Chapter 1 

Theory and Application of the 
Electromagnetic Borehole Flowmeter 


1-1 Overview 

The underestimation of aquifer heterogeneity has significantly 
contributed to the inefficient design and, consequently, 
inadequate performance of many types of remediation systems 
(Mercer et al., 1990; Haley et al., 1991). This is especially 
true of pump-and-treat remediation systems. In many cases, 
interpretation of hydraulic properties is limited by field 
measurement capability. Characterization of a hydrogeologic 
system requires an effective method for measuring vertical 
variations in hydraulic properties. At least three research 
teams (Boggs et al., 1989; Rehfeldt et al., 1989b; Molz et al., 
1989, 1990) have evaluated alternative methods for measuring 
the vertical variation of hydraulic conductivity (K). These 
methods included: small-scale tracer tests, multilevel slug 
tests, laboratory permeameter tests, empirical equations based 
on grain-size distributions, and borehole flowmeter tests. 
These comparisons indicated that the borehole flowmeter test 
is one of the most promising methods for measuring the 
spatial variability of the hydraulic conductivity fields. 

Various types of flowmeters based on impeller, thermal-pulse, 
tracer-release or electromagnetic technologies have been 
devised for measuring flow distribution along a borehole or 
well screen. Impeller meters (also known as spinners) have 
been used for several decades in the petroleum industry, and 
such instruments suitable for some ground-water applications 
are now available. A meter of this type was applied in the 
field by Hufschmied (1983), Molz et al. (1990), and others. 
Most impeller meters cannot operate at the lower flow ranges 
often required for ground-water applications. Based on the 
need to measure low flow velocities, the United States 
Geological Survey (USGS) developed a thermal-pulse 
flowmeter (Hess, 1982, 1986; Morin et al., 1988a; Paillet et 
al., 1987). Such tools are quite sensitive to low flow 
velocities. Electromagnetic flowmeters can also operate 
within a relatively large range of flow rates suitable for most 
ground-water investigations. Although this document focuses 
on the design and application of the electromagnetic 
flowmeter developed under this project for the measurement 
of the vertical component of flow within a well, the data 
collection and methods of analysis apply to other types of 
vertical component flowmeters. 

A sensitive, vertical-component borehole flowmeter enables 
one to accomplish two basic tasks: 

1. It allows one to measure the natural (ambient) 
vertical flow that exists in many wells; and 


2. If the well is pumped at a steady rate, it enables one 
to determine the flow distribution that is entering the 
well from the surrounding medium. 

Ambient flow distributions provide information on the 
direction of the vertical component of the hydraulic gradient, 
and on the location of hydraulically active fractures in the case 
of a fractured formation. If certain conditions are met, flow 
distributions during pumping provide information on the 
relative differences in the permeability of selected aquifer 
zones or additional information on fracture hydraulic 
characteristics. 

Flowmeter measurements depend on factors such as skin 
effects due to well construction and development and ambient 
hydrologic conditions. Such factors may be variable within a 
well and some may change with time. Also, it cannot be 
overemphasized that a borehole flowmeter should be viewed 
only as another tool available to ground-water hydrologists. 

As expected, the best hydrogeologic characterizations are 
achieved when this tool is used in combination with other 
methods (e.g., geophysical measurements and traditional 
aquifer tests). Application of several technologies that are 
suitable for characterization of various aspects of site 
hydrology on different scales are necessary for definition of 
hydrogeologic controls on ground-water flow. 

A sensitive borehole flowmeter, such as tools based on 
thermal-pulse technology or the electromagnetic system 
discussed in this report, is most applicable for characterization 
objectives that require few assumptions regarding aquifer 
properties or other complex variables. These tools are directly 
applicable for such objectives as identifying transmissive 
intervals in wells, evaluation of the effects and state of well 
development, and identification of natural flow patterns within 
conventional monitoring wells. Information regarding zones 
where ground water enters and, in the case of ambient flow, 
exits a well is often essential for detailed evaluation of 
ground-water monitoring efforts. Studies (e.g., Church and 
Granato, 1996; Collar and Mock, 1997; Martin-Hayden and 
Robbins, 1997) have demonstrated the potential effects of the 
mixing of waters with different chemistries within a 
conventional monitoring well on contaminant transport 
evaluations. Borehole flowmeters provide direct information 
concerning the mix of water that enters a well under either 
ambient or pumping conditions. In fractured rock settings, 
where water may enter the well from only a few discrete 
intervals, the flowmeter allows identification of those 
intervals. Such intervals may then be individually targeted for 




characterization of water chemistry. Direction of natural flow 
within a well under ambient conditions is a direct means of 
assessing vertical components of the hydraulic gradient at the 
time of the study. This information is useful in conceptu¬ 
alization of the hydrologic setting in various parts of a site. 
Such direct uses of these tools are readily apparent and not 
discussed in detail in this report. 

These tools may also provide data used in the estimation of 
detailed profiles of the differences in hydraulic conductivity of 
granular aquifer materials. However, many simplifying 
assumptions are required in such analyses increasing 
uncertainty in the results as is true for more traditional aquifer 
tests. This indirect use of data obtained using these tools is 
not readily apparent and is discussed in detail in this 
document. As with any site characterization tool, study 
objectives and site conditions must be critically evaluated 
during test selection and design. Results obtained from 
borehole flowmeter studies, such as investigations described 
in this document, should be viewed as only one piece of 
information used in the overall characterization of site 
conditions. 

1-2 Principles of Operation 

An electromagnetic (EM) prototype flowmeter (Figure 1-1) 
was developed at the Tennessee Valley Authority (TVA) 
Engineering Laboratory in Norris, Tennessee. It consists of an 
electromagnet and two electrodes that are cast in a durable 
epoxy. The epoxy is molded in a cylindrical shape to 
minimize turbulence associated with channeling water past the 
electrodes and electromagnet. Having no moving parts, the 
flowmeter operates according to Faraday's Law of Induction, 
which states that the voltage induced across a conductor 
moving at right angles through a magnetic field is directly 
proportional to the translational velocity of the conductor. 
Flowing water is the conductor, the electromagnet generates a 
magnetic field, and the electrodes are used to measure the 
induced voltage. Electronics connected to the electrodes 
transmit a voltage that is directly proportional to the velocity 
of the water. 

1-3 Collection of Flowmeter Logs 

The concept of borehole flowmeter measurements using the 
simplest test design is illustrated in Figure 1-2. A flowmeter 
log is recorded before pumping to measure any ambient flow 
in the well. This step is very important, particularly in the 
case of highly sensitive flowmeters and low permeability 
formations, where the ambient flow may be a significant 
fraction of the flow induced by pumping. Following the 
ambient test, a pump is placed in the well and operated at a 
constant flow rate, QR After a steady flow field toward the 
well is obtained, the flowmeter is positioned near the bottom 
of the well and a measurement of discharge rate is obtained. 
The meter is then raised a distance Az and another reading is 
taken. As illustrated in Figure 1-2, the result is a series of 
measurements of cumulative vertical discharge, Q, within the 
well screen as a function of vertical position, z. Just above the 


top of the screen the meter reading should be equal to QP, the 
steady pumping rate that is measured independently at the 
surface. This procedure may be repeated several times to 
ascertain that the readings are stable and flow to the well has 
reached a steady-state condition. EM flowmeters are capable 
of measuring upward or downward flow. Therefore, if the 
selected pumping rate, QP, causes excessive drawdown or 
there are concerns associated with disposal of contaminated 
ground water, one can employ an injection procedure as an 
alternative. 

1-4 Analysis of Flowmeter Logs 
a. Flow Distribution Logs 

The basic data obtained in the field (Table I-1) are: 

• Column (a) - the elevations where readings are 
taken, 

• Column (b) - the ambient flow log, and 

• Column (d) - the total flow log, which is measured 
flow under pumping conditions. 

The following data are then calculated: 

• Column (c) - differential ambient flow, 

• Column (e) - the net flow log, and 

• Column (f) - the differential net flow. 

The data in Table 1-1 are shown graphically in Figure 1-3. The 
aquifer base is located at z = 0, and the water table or upper 
confining layer is located at z = 20 m. 

Measurements and calculations are made based on the 
assumed layered geometry illustrated in Figure 1-4. Flow to 
the well is assumed to be horizontal. The basic flow logs, 
Columns (b) and (d) represent upward (positive) or downward 
(negative) vertical flow within the well itself; while the 
differential flow logs represent horizontal flow in the assumed 
layers to the well (positive) or from the well (negative). The 
sign convention introduced herein is not universal. However, 
the same sign convention should be followed for any 
particular application. 

For the example of Table 1-1 and Figure 1-3, the ambient flow 
is upward. The differential ambient flow log, obtained by 
taking differences between adjacent values of ambient flow in 
the well, indicates that water enters the well at varying rates 
from the bottom half of the aquifer and exits the well at 
varying rates in the top half of the aquifer. 

At this stage in the data analysis, a distinction must be made. 
The drawdown that is measured while recording the total flow 
log is due only to pumping. However, the measured flow log 
is due to a superposition of the ambient flow and the pumping 
flow. Therefore, to be consistent one must subtract the 
ambient flow log from the total flow log to obtain the net flow 
log, which is that portion of the total flow due to the measured 
drawdown. If this subtraction is not made, the results could be 
ambiguous. 


2 





Top View 
Section AA 


Side View 
Section BB 


Amplifier 

Electromagnet 


Electrodes 
Iron Core ^ 




B Magnetic Field 
E Induced EM Field 
v Velocity of Fluid 


Figure 1-1. Schematic diagram of the TVA electromagnetic borehole flowmeter. 


QP <- 


(Discharge from Pump) 
---- 


Pump 


To Flowmeter Logger (Q) 




V 


< 



Screen 

or 

Borehole 


Borehole 

Flowmeter 


I 

I 

I 

I 

I 

-> 


Casing 



Figure 1-2. Apparatus and geometry associated with a borehole flowmeter test. 


3 























































Ambient Flow - Column (b) Data Differential Ambient Flow - Column (c) Data 



4 


Figure 1-3. Graphical illustration of the hypothetical case presented in Table 1-1. 




































+ 

N 

II 

N 


N 

II 

N 


O 


+ 

o 


o 


+ 

o 



N 



t 


c. 
CD 

E 

CT) 
O CD 
COCO 


c 

CD 

CD 


o 

<1 


CM CO 

■ i 

c c 


CO CM 


5 


Figure 1-4. Assumed layered geometry within which flowmeter data are collected and analyzed. 

























































































Table 1-1. Hypothetical Data and Data Analysis Related to the Application of a Borehole Flowmeter 


Elevation 

(m) 

Ambient Flow 
(L/min) 

A Ambient Flow 
(L/min) 

Total Flow 
(L/min) 

Net Flow 
(L/min) 

r 

ANet Flow 
(L/min) 

(a) 

(b) 

(c) 

(d) 

(e) 

( 

(f) 

20 

0.0 

-.07 

6.0 

6.0 

0.87 

18 

0.07 

-0.10 

5.2 

5.13 

0.70 

16 

0.17 

-0.16 

4.6 

4.43 

1.16 

14 

0.33 

-0.37 

3.6 

3.27 

1.27 

12 

0.70 

-0.05 

2.7 

2.00 

0.25 

10 

0.75 

+0.15 

2.5 

1.75 

0.55 

8 

0.60 

+0.21 

1.8 

1.20 

0.19 

6 

0.39 

+0.21 

1.4 

1.01 

0.19 

4 

0.18 

+0.13 

1.0 

0.82 

0.57 

2 

0.05 

+0.05 

0.3 

0.25 

0.25 

0 

0.00 

— 

0.0 

0.00 

— 


The final column in Table 1-1, the differential net flow, is 
obtained by calculating the difference between adjacent values 
in the net flow log. This yields the horizontal flow that is 
entering the well from each interval due to pumping. Under 
conditions of steady, horizontal flow into the well, this flow is 
proportional to the vertical distribution of horizontal hydraulic 
conductivity in the vicinity of the test well (Molz et al., 1988; 
1989). 


vertical distribution of hydraulic parameters. Such a 
procedure does not require knowledge or estimation of 
specific storage (S s ) and may have advantages in certain 
situations where S s is expected to vary greatly. Only the first 
two methods are discussed herein. Xiang (1995), Hanson and 
Nishikawa (1996), and Ruud and Kabala (1996) provide 
additional discussion on numerical evaluation of flowmeter 
tests. 


b. Hydraulic Conductivity Logs in Granular 
Aquifers 

For purposes of these analyses, the aquifer is assumed to be 
composed of a series of n horizontal layers (Figure 1-4). In 
practice, hydrogeologic information, such as data available 
from cores and geophysical logs, may be used to 
conceptualize potential geologic layers and choose appropriate 
intervals for measurements during the flowmeter study. The 
difference between two successive flowmeter readings 
obtained under pumping conditions yields the flow, AQj, 
entering a well screen between the elevations where the 
readings are taken, which are assumed to bound layer i (i = 
l,2,...,n). The Aqj from the ambient flow log are computed in 
an identical manner. Most, aquifers are not composed of 
horizontal, homogeneous layers. However, the n-layer case at 
this scale is more realistic than the single-layer case of 
classical pumping test analyses. 

To calculate a hydraulic conductivity profile, three methods 
have been detailed in the literature. The first is based on the 
Cooper-Jacob equation relating drawdown to a constant 
pumping rate in a fully penetrating well (Cooper and Jacob, 
1946). The second is based on the numerical results by 
Javandel and Witherspoon (1969). Molz and Young (1993) 
provided an overview of these methods. A third method is the 
two-step procedure described by Kabala (1994), which yields 
an Sj as well as a K distribution. Drawdown data collected at 
different times and flowmeter data are used to estimate the 


In the Cooper-Jacob method, the assumed horizontal flow in 
each layer is treated as if it was from an aquifer of infinite 
horizontal extent and thickness, Azj. Then for each layer, i, 
one can write 


mv) = 


m- h) 

2nKAz- 

I I 




( 1 - 1 ) 


where: 


AH, = 

drawdown in ith layer 

AQ, = 

induced flow from ith layer 

A qi = 

ambient flow from ith layer 

Ki = 

horizontal hydraulic conductivity of the ith 
layer 

Azj = 

ith layer thickness 

r w 

effective well radius 

t 

time since pumping started 

s, 

storage coefficient for the ith layer. 


If one assumes that head losses associated with flow within 
the well are negligible, then all the AHj are equal to AH, the 
measured drawdown in the test well. If such an assumption 
cannot be made, then one must measure the head loss opposite 
each layer associated with pumping the test well. Rehfeldt et 
al. (1989b) provide further discussion of various possible head 
losses. 


6 











Often, a value for the storativity of the aquifer being studied 
will be estimated, and the question becomes how to use this 
information to obtain Sj for each layer. In previous studies, 
two assumptions have been made. The most basic assumption 
is to assume that S s , the specific storage, is constant, in which 
case Sj = S s AZj (Morin et al., 1988a; Molz et al., 1989). An 
alternate assumption used by Rehfeldt et al. (1989b) is that S s 
varies in such a way that the hydraulic diffusivity of each 
layer, KjAz/Sj, remains constant and equal to the hydraulic 
diffusivity (T/S) for the aquifer, where T is transmissivity and 
S is the storage coefficient. If the latter assumption is made, 
Equation (1-1) may be solved for Kj yielding 


K = i n 

15 J T ‘ 

‘ 2 KAHAz t 

r 1 5 

w 1 


( 1 - 2 ) 


Finally, substituting for a in Equation (1-4) and solving for 
K gives 


K 

l 

~K 


(AQ - Aq^/Az; . f „ 

- -*■ - \i-l,2,...n . 

QP/b 


(1-7) 


To obtain Equation (1-7), it was assumed that AQj and QP do 
not change with time and pseudo-steady-state conditions were 
reached. Thus, a plot of Kj/K versus elevation may be 
obtained from the basic data. If one then has an estimate of 
K from a conventional aquifer test (e.g., fully-penetrating 
pumping test), dimensional values for Kj can easily be 
calculated by taking the product of K and the flowmeter test 
result. 


If the constant S s assumption is made, one can solve Equation 
(1-1) for Kj outside the logarithmic term to yield 



(AQ- H) 

2kAHAz, 


1.5 

1 K Az i t 

r 

w 

1 . 


d-3) 


which can be solved iteratively to obtain a value for Kj. 
Further details may be found in Morin et al. (1988a), Molz et 
al. (1989), or Rehfeldt et al. (1989b). 


In situations where the inherent assumptions are valid, the 
Kj/ K approach has practical appeal because the values for r w 
and Sj, which are difficult to estimate, are not required. Also, 
errors in flowmeter readings involving constant multipliers are 
canceled out, and the meter calibration is not critical as long as 
its response is linear. However, a reliable aquifer test must be 
performed to estimate K . A detailed example of data analysis, 
including head loss measurements for each layer, is presented 
in the following section. 

c. Data Analysis 


Javandel and Witherspoon (1969) showed that, in idealized 
layered aquifers, flow at the well-bore radius, r w , rapidly 
becomes horizontal even for relatively large permeability 
contrasts between layers. Under such conditions the radial 
gradients along the wellbore are constant and uniform, and 
flow into the well from a given layer, due to pumping, is 
proportional to the transmissivity of that layer, that is: 


The data presented in this section were obtained from a 
borehole flowmeter test conducted in a fully-penetrating well 
in a confined aquifer located 40 m to 61 m below ground. The 
well screen was 10.2-cm diameter slotted PVC pipe (0.025-cm 
slots). Testing began with mild redevelopment and cleaning of 
the test well screen using injected air. 


(AQ- Aq ,)= aAz,K, (1-4) 

where a is a constant of proportionality. This condition occurs 
when the dimensionless variable t D = Kt/(S s r*)is > 100. 

In this expression, K is the average or bulk horizontal 
hydraulic conductivity defined as XKjAz/b, where b is the 
aquifer thickness, S s is the aquifer specific storage, t is time 
since pumping started, and r w is the wellbore radius. 

To solve for a, sum the (AQ r Aqj) over the aquifer thickness, 
to obtain 

i (AQ r Aq.)= QP = a I Az,K, . (1-5) 

1=1 z=l 


Prior to the flowmeter survey, caliper and ambient pressure 
logs were run. Data obtained from the caliper logs were used 
to verify and compute the cross-sectional area of the well, and 
the hydraulic head distribution derived from the pressure logs 
served as calibration references for evaluating AH, produced 
by pumping. No ambient flow was detected. Subsequently, a 
pressure transducer and a flowmeter with centralizer were 
lowered into the well, followed by a submersible pump. The 
pump was started and allowed to run for about 50 minutes 
prior to taking pressure and impeller meter readings. 

Listed in the first three columns of Table 1-2 are corresponding 
values of depth, head change, and discharge associated with 
pumping. This constitutes the basic data resulting from the 
flowmeter test. The discharge measured at the 40-m depth 
(0.24 m 3 /min) was taken as the pumping rate QP. 


Multiplying the right-hand side of Equation (1-5) by b/b and 
solving for a yields 


a- 


QP 
b K 


d-6) 


Additional hydraulic information about the aquifer in the 
vicinity of the test well was obtained from small-scale 
pumping tests. A standard Cooper-Jacob (1946) analysis 
resulted in a transmissivity of 0.83 m 2 /min, a K of 0.039 m/ 
min, a storage coefficient of 4.5 x 10' 3 , and a specific storage 
of 2.1 x 10‘ 4 nr 1 . Data analysis proceeds by subtracting 


7 

















Table 1-2. Results of the Analysis of Data from a Borehole Flowmeter Test to Estimate Hydraulic Conductivity Distributions 


z(m) 

AHi(m) 

Q(m 3 /min) 

AQi(m 3 /min) 

K2;(m/min)‘ 

K3i(m/min)* 

K7|(m/min)‘ 

Interval (i) 

39.6 

0.371 

0.240 

0.0062 

0.014 

0.013 

0.014 

13 

41.1 

0.366 

0.234 

0.0057 

0.013 

0.012 

0.012 

12 

42.7 

0.362 

0.229 

0.0147 

0.034 

0.033 

0.033 

11 

44.2 

0.359 

0.214 

0.0071 

0.016 

0.015 

0.016 

10 

45.7 

0.355 

0.207 

0.0119 

0.027 

0.027 

0.027 

9 

47.2 

0.353 

0.195 

0.0057 

0.013 

0.012 

0.013 

8 

48.8 

0.350 

0.189 

0.0113 

0.027 

0.026 

0.026 

7 

50.3 

0.348 

0.178 

0.0422 

0.099 

0.105 

0.098 

6 

51.8 

0.347 

0.136 

0.0153 

0.036 

0.036 

0.035 

5 

53.3 

0.347 

0.120 

0.0099 

0.023 

0.023 

0.023 

4 

54.9 

0.347 

0.110 

0.0331 

0.078 

0.082 

0.077 

3 

56.4 

0.347 

0.077 

0.0436 

0.103 

0.109 

0.101 

2 

57.9 

0.347 

0.034 

0.0337 

0.040 

0.042 

0.039 

1 

61.0 

0.347 

0 






‘Based on Equations 1-2,1-3, and 1-7, respectively. 


neighboring Q values to get the AQ, for each of the 13 layers 
(Column 4, Table 1-2), noting that layer 1 is 3.05-m thick and 
succeeding layers are 1.52-m thick. Listed in the last three 
columns of Table 1-2 are K distributions denoted by K2, K3, 
and K7. These were calculated based on Equations (1-2), 
(1-3), and (1-7), respectively. In order to account for the small 
change in pressure head between the bottom and top of the 
screen, Equation (1-7) was modified to read 


K _ AH(AQ- Aq.)/A Zi 


K 


AH . QP/b 


; i = 1, 2, ...n. 


d-8) 


Equation (1-8) is a good approximation when the AHj are close 
to AH, the average change in pressure head. When the 
aquifer/well hydraulic data, including r w = 0.0634 m and 
t = 60 min, are substituted in Equations (1-2), (1-3), and (1-8), 
one arrives at (meter, minute units) 


K2 = 


0.8176 AQ t 
AH 


0.1044 AQ In (12,677 Ik3.) 

K3 ‘= - AH - a " d 


K7 = 


0.8031 AQ i 
AH 


d-9) 


with the exception of the bottom layer where Az, = 3.05 m 
rather than 1.52 m. The results show that there are no 
significant differences between the three alternate hydraulic 
conductivity calculations using data from this flowmeter test. 


1.5 Conclusions 

Presented in this chapter is an overview of vertical-component 
flowmeter operation, application, and data analysis. While the 
EM flowmeter was emphasized, the data collection and 
analysis procedures apply to many types of flowmeters. A 
flowmeter of this design measures only one thing directly: 
upward or downward water velocity in a borehole, screen, or 
casing due to natural head differences (ambient flow) or to 
artificial head differences (pumping-induced flow). By 
calibrating the meter for volumetric flowrate and taking 
differences between two vertical flow measurements at known 
elevations, assuming that no undetected flow is leaking past 
the meter, it is possible to calculate the incremental horizontal 
flow in the aquifer to or from the well. It is the horizontal 
flow distribution over the vertical extent of the well that 
enables one to calculate a horizontal hydraulic conductivity 
distribution as a function of position in the vertical, K(z). The 
calculated K distribution may be absolute, K(z), or relative, 
K(z)/ K . Alternate formulas are given for both quantities. 

The advantage of the K(z)/ K calculation [Equation (1-7)] is 
that well radius and storativity do not have to be known or 
estimated. 


8 












Chapter II 

Electromagnetic Borehole 
Flowmeter System 


II-l Design and Construction 

The electromagnetic borehole flowmeter system discussed in 
this report can measure vertical flow in various types of wells 
and boreholes. The flowmeter system has three main 
components: downhole flowmeter, packer assembly, and 
above-ground electronics. The flowmeter provides accurate 
flow measurements across a four order-of-magnitude range 
and fits snugly in wells with casing diameters as small as 
5.1 cm (standard 2-inch schedule-40 PVC pipe). A packer 
assembly may be attached to the flowmeter to direct flow 
through the meter in larger diameter wells. The above-ground 
electronics package provides the magnetic drive and converts 
the flowmeter signal to a discharge reading. 

Two of the major components of the meter are an 
electromagnet and a pair of silver chloride electrodes mounted 
at right angles to the pole pieces of the electromagnet (Figure 
I-1). During operation, the electromagnet creates a strong 
magnetic field across the flow passage. As water (the 
conductor) flows through the magnetic field, a voltage 
gradient is generated. The voltage is proportional to the 
average velocity of the water across the magnetic field and is 
detected by the electrodes. The magnitude of the voltage is 
unaffected by the electrical conductivity of ground water 
under normal conditions. The polarity of the generated 


voltage is dependent on the direction of flow. Upward flow is 
generally designated as a positive voltage and downward flow 
as a negative voltage. 

Flowmeters of this design can be built to any outer diameter 
greater than 4.8 cm. As flow is channeled through this meter, 
the applicable flow range is a function of the inner diameter of 
the open probe core. Flowmeters with inner diameters (IDs) 
of 1.27 cm or 2.54 cm have been produced. The length of the 
current prototype flowmeter is 30 cm, but flowmeters as short 
as 10 cm have been used successfully. The 1.27-cm and 2.54- 
cm ID flowmeters are typically used to measure low and high 
flow rates, respectively. In wells or boreholes with relatively 
large diameters, the effectiveness of the flowmeter diminishes 
unless a packer assembly is used to direct flow through the 
flowmeter. Both mechanical (Figure II-1) and inflatable 
(Figure II-2) packers have been developed. 

The mechanical packer assembly used with prototype meters 
was a collar consisting of a rubber gasket sandwiched between 
two plexiglass or stainless steel rings that slipped over the 
flowmeter and was held in place with set screws. The rubber 
gasket was sized to insure a tight seal between the probe and 
the inner surface of the well. The mechanical packer was 
easily used, but its application was tedious because the friction 
between the collar and well made lowering of the flowmeter 



Retaining Rubber Gasket 



Bolts (4) 


(a) 


(b) 


Figure 11-1. Mechanical packer (a) assembled on probe and (b) in schematic view. 


9 


















Clamps 



Figure 11-2. Inflatable packer assembled on probe. 


more difficult. In situations where the screen or borehole was 
uneven, the collar design may not have provided an adequate 
seal and was not used. 

The inflatable packer for the prototype system consisted of a 
rubber sleeve attached to a stainless steel assembly. The 
packer assembly easily slipped onto the flowmeter and sealed 
with “O” rings. This assembled unit had a diameter of 8.9 cm. 
If the rubber sleeve was slipped directly onto the flowmeter, 
the flowmeter and sleeve had a combined diameter of 8.7 cm. 
Approximately 103 kPa (15 psi) and 172 kPa (25 psi) of 
pressure was needed to inflate the rubber sleeve to diameters 
of 10 cm and 20 cm, respectively. Inflation was achieved by 
injecting water into the rubber sleeve through a tube in the 
packer assembly. Both a pressurized chamber at the ground 
surface and a submersible pump have been used to inject 
water into the sleeve. For most applications, the submersible 
pump was the method of choice. However, at shallow depths 
(<20 meters) where only a few flowmeter measurements were 
desired, the pressurized chamber had an advantage in its 
simplicity and relatively short set-up time. The seal provided 
by a properly inflated packer was such that the flowmeter 
could not be moved by pulling the cable and rope. 

The above-ground electronics includes an electromagnet drive, 
power supplies, amplifiers, and synchronous demodulator for 
converting the voltage from the probe’s electrodes to flow 
rate. The probe’s signal is in the microvolt range and is 
typically several orders-of-magnitude less than background 
noise. Synchronous demodulation is used to extract the signal 
and has the effect of canceling noise out of phase with the 
electromagnetic drive. With additional amplification and 
filtering, a DC signal proportional to water velocity through 
the flowmeter is generated. The electronics package collects 
and processes signals from the flowmeter every second. At 


the end of a pre-set time interval or upon keyboard command, 
the signals are averaged and the standard deviation is calcu¬ 
lated. This average and standard deviation are displayed, 
stored on disk, and printed to a hardcopy device. 

II-2 Laboratory Calibration 

Calibration of the prototype flowmeter was performed in a 
facility located at the TVA Engineering Laboratory, Norris, 
Tennessee. The apparatus included standard 5.1-cm, 10.2-cm, 
and 15.2-cm schedule-40 flush-joint PVC pipes. Before and 
after any field measurements, calibration checks were 
performed at a range of flow rates using the same flowmeter 
system (above-ground electronics, cable, and flowmeter) used 
in the field. Calibrations were simple and consisted primarily 
of establishing a constant uniform flow through a vertical PVC 
pipe and comparing the flowmeter measurements to other flow 
measurements at the intake and/or the outlet of the PVC pipe 
(Figure II-3). Flow rates were maintained by throttling a 
pressure valve on the public waterline and measuring the 
generated flow rate near the PVC pipe intake with an in-line 
commercial flowmeter. For all flows, the baseline rate was 
determined by measuring the time required for the discharge 
to fill a calibrated container. At lower flow rates (< 30 ml/ 
min), the discharge volume was determined by dividing the 
weight of water by the temperature corrected density of water. 

Electromagnetic flowmeter calibrations are similar to those for 
other flowmeter types and require: (1) the use of proper and 
certified measuring devices for volume, time, length, weight, 
and temperature; and (2) the documentation of calibration 
conditions including personnel, date and time, and equipment 
set-up. Instructions and guidelines for performing flow 




Well Casing 



Flowmeter 



s 


X3 


t Water Supply 


Calibrated Vessel 


Figure 11-3. Conceptual system for flowmeter calibration. 


10 












































calibrations can be found in standard texts such as Ferson 
(1983) and Liptak and Venczel (1982). Besides standard 
flowmeter calibrations, similar equipment has been used to 
investigate phenomena such as turbulence, frictional losses, 
and flow around the probe. The three main PVC pipes used in 
the calibration facility are transparent to permit visual 
monitoring of dye releases so that turbulence and flow around 
the probe can be evaluated. One of the PVC pipes is fitted 
with manometers to measure head losses within the pipe and 
flowmeter. All of the PVC pipes possess side ports to permit 
the introduction of flow. This allows monitoring of the 
flowmeter response as it approaches and passes a horizontal 
inlet source. Horizontal inflows may be large enough to 
produce turbulence effects, and gauging the sensitivity of the 


flowmeter to these inflows was important. Testing indicates 
that the electromagnetic flowmeter is insensitive to the 
proximity of horizontal inflows. 

Figure II-4 provides a sample calibration data set associated 
with field testing of the EM flowmeter. The figure shows the 
calibration data on both linear and logarithmic scales. The 
linear scale illustrates the linear response between volts 
(flowmeter signal) and flow. The logarithmic scale shows the 
sensitivity of the meter at lower flow ranges. The 2.54-cm ID 
flowmeter exhibits good repeatability and linearity from about 
100 ml/min to 40 L/min. The 1,27-cm ID flowmeter exhibits 
good repeatability and linearity from 30 ml/min to 10 L/min. 
Below 30 ml/min, the repeatability and linearity of the 


Calibration 

1.27-cm ID Flowmeter in 5.1-cm PVC Pipe 


10 



*• 

5 


• - 5-13-92 

* * 

• 

2 


• - 5-27-92 

*• 



* - 6-29-92 

- 

1 


X - 7-08-92 

• 

* 

Its 

o 

cn 

- 

□ - 7-10-92 

* 

o 

> 


X - ± 1 Standard 

*• 

0.2 


Deviation 

* 1 

0.1 



**** 

0.05 


> 

*** 

0.02 

- 



0.01 


* 

X 1 * 

! 1 _1_!_f ' ■ J-iJ-!-1... 


0.01 0.1 1 10 

Discharge (L/min) 


w 

o 

> 


10 


• - 5-13-92 

• - 5-27-92 

• - 6-29-92 

X - 7-08-92 

□ - 7-10-92 

I - ±1 Standard 
Deviation 


10 


12 


14 


Discharge (L/min) 


Figure 11-4. Calibration data for the 1.27-cm ID flowmeter in 5.1-cm PVC pipe. 


11 














1,27-cm ID flowmeter drops significantly. Deviation from 
linearity at these low flows may be a result of shifts in the 
flowmeter response over time, or problems/uncertainties 
associated with maintaining and measuring low flows during 
calibration. 

In Figure II-4, each calibration point is the mean of sixty 
readings taken over a one minute period. For the majority of 
the flow measurements, the relative standard deviation is less 
than 2 percent of the average flow rate. Significantly lower 
percentages occur at the higher flow rates and slightly higher 
percentages occur at the lower flow rates. Calibration data 
show that the sensitivity and low end accuracy (below 50 ml/ 
min) of the 1.27-cm ID flowmeter is less in a 5.1-cm pipe 
without a packer assembly, than in larger diameter pipes with 
a packer assembly. For the 5.1-cm pipe, the calibration plot is 
essentially linear up to 6 L/min. Above 6 L/min, the 
flowmeter signal becomes slightly nonlinear. As no packer 
assembly was used in the 5.1-cm application, the nonlinearity 
is probably the result of water flow around the probe. 

Flowmeter sensitivity can be expressed in a calibration factor. 
Calibration factors are used to convert voltage from the probe 
electrodes to flow and have the units of volume per time per 
volt (e.g., liter/minute/volt). Table II-1 provides example 
calibration factors calculated for different combinations of 


Table 11-1. Sample Calibration Factors (Liter/M in/Vo It) from a 

Linear Regression of Discharge Versus Voltage for 
Electromagnetic Flowmeter Data 


1.27-cm ID 

EM Flowmeter 

2.54-cm ID 

EM Flowmeter 

Flowmeter in a 5.1-cm pipe* 

1.46 

3.99 

Flowmeter with a mechanical 



collar in a 10.2-cm pipe* 

1.38 

3.95 

Flowmeter with an inflatable 



packer in a 15.4-cm pipe* 

1.32 

3.94 


* pipe is schedule-40 PVC 


flowmeter sizes, packer types, and well diameters. Greater 
sensitivity is achieved with a packer because less water flows 
around the exterior of the flowmeter. The flowmeter should 
be calibrated using the same equipment and in the same type 
of casing as the wells to be surveyed prior to use. 

At flow rates above 10 L/min, two concerns exist with use of 
the 1.27-cm ID flowmeter: high frictional losses through the 
orifice, and an electrode voltage that will exceed the capacity 
of the above-ground electronics. High frictional losses (Figure 
II-5) affect the hydraulic head distribution and, consequently, 
the distribution of flow in the well and through the meter. In 
selecting between electromagnetic flowmeters with different 
orifice diameters, the trade-offs associated with decreases in 
the detection limit and increases in frictional losses should be 
considered. As a general rule, the frictional losses should be a 
concern if they exceed 10 percent of the total drawdown in a 
well or borehole for cases where the data will be used to 
calculate a hydraulic conductivity profile. 



Figure 11-5. Frictional losses associated with the 1.27-cm and 
2.54-cm ID flowmeters. 


12 












Chapter III 

Hydrogeologic Characterization Test Design 


III-l Overview 

The primary objective of an electromagnetic borehole 
flowmeter test is to measure the profile of horizontal flow into 
or out of designated aquifer intervals during ambient and/or 
pumping conditions. These profiles are used to identify the 
zones of relatively high and low permeability. Combined with 
a transmissivity measurement or the proper drawdown data at 
the tested well, the flowmeter data may be used to calculate a 
vertical profile of horizontal hydraulic conductivity. In many 
situations, the pumping tests performed concurrently with 
such flowmeter tests will provide the drawdown data 
necessary to calculate a transmissivity value. 

Table III-1 lists the general procedures which were used in 
these studies to perform borehole flowmeter tests. Detailed 
descriptions of these procedures and associated equipment are 
contained in the following sections. Appendix A provides the 
forms used to record the field data and Appendix B provides 
an equipment checklist. 

III-2 Measurement Interval Selection 

Selection of appropriate intervals for obtaining flowmeter 
measurements in porous media involves consideration of site 
stratigraphy interpreted from geologic and geophysical logs. 
However, such logs generally provide only enough 
information to determine the basic hydrogeologic framework 
and not the detailed variability within geologic units. 
Although, these logs may provide constraints for specifying 
measurement intervals, the scale on which measurements are 


ultimately obtained will usually be based on other factors. 

Such factors include the pumping rate to be used during the 
test. Constraints on pumping rate, such as limitations related 
to water storage, treatment, and disposal, may necessitate 
increased interval size to provide meaningful data. The 
thickness of the disturbed zone surrounding the well screen is 
also a factor that affects interval selection. Results of studies 
by Ruud and Kabala (1997) indicate that skin effects bias the 
distribution of flow to a well. As the ratio of the flowmeter 
measurement interval thickness to the thickness of the 
disturbed zone decreases, the bias in the test results increases. 
In general, local deviations from horizontal flow will occur 
during most tests. Errors associated with these deviations will 
increase as the size of the measurement interval decreases. 

III-3 Flowmeter Measurements 

Prior to testing, the flowmeter is connected to the electronics 
and permitted to warm up. This warm-up period was 
approximately 30 min for the prototype electromagnetic 
system. Well construction logs, including a caliper log of the 
test well, are reviewed if there is any doubt about the screen or 
borehole diameter. Field testing has shown that the calibration 
of the prototype electromagnetic flowmeter may drift up to 
10 millivolts (0.1 percent full scale) over several hours if large 
temperature changes (> 10°C) occur. In many instances, this 
temperature sensitivity is not a concern because of the 
relatively short time needed to obtain a flow log. However, 
flowmeter calibration at zero flow is checked prior to initiation 
of the test and at the end of each test. This is accomplished by 
positioning the flowmeter at a location in the well where no 


Table 111-1. Generalized Borehole Flowmeter Field Test Procedures 


1. Measure ground-water elevation in the well relative to a fixed datum (such as top of well casing) and total well depth. Examine well 
construction, geologic, and geophysical logs to determine measurement intervals. 

2. Zero flowmeter under no-flow conditions. Measure the ambient vertical flow profile. In these studies, this profile typically consisted of a 
minimum of ten measurements. Obtain flowmeter reading under no-flow conditions to evaluate baseline drift. 

3. Install equipment as for a single-well pumping test, including pressure transducer, data logger, and pump. Place intake hose or pump 
above the pressure transducer and within the well casing above the screen, if possible. Measure ground-water elevation to insure static 
conditions are attained. 

4. Start constant-rate pumping test. Accurately measure pump discharge rate on a routine basis. Record drawdown. Verify that wellbore 
storage effects have dissipated and estimate transmissivity based on drawdown data. 

5. After pseudo-steady-state conditions have been achieved, obtain flowmeter measurements at the same elevations as the ambient 
measurements. Repeat flowmeter measurements to demonstrate a stable flow profile has been achieved. Shut system down and obtain 
flowmeter reading under no-flow conditions. 


13 





ambient flow exists. In a well, no ambient flow should exist 
above or below the screened interval. In a borehole, no 
ambient flow should occur in the cased interval. If there is 
doubt about the location of a no-flow zone, the problem is 
resolved by manually plugging the flowmeter and lowering it 
into the well. Plugging may be accomplished by securely 
fastening a cork in the lower orifice of the flowmeter. 

Lowering or raising the flowmeter induces water movement in 
a wellbore and it may require several minutes to regain 
quiescent conditions in low permeability aquifers. After 
moving the flowmeter, measurements should not begin until 
the flowmeter signals have stabilized. If the readings do not 
stabilize, and/or if the standard deviation is approximately ten 
times greater than observed in the calibration data for the 
measured flow, then reject the reading and attempt to diagnose 
the problem. Table III-2 lists problems that have been 
encountered in the field. Problems 3 and 4 can often be 
remedied by rapidly moving the flowmeter up and down the 
well or borehole several times. 

Table 111-2. Problems that Produce Errors in EM Flowmeter 
Measurements 


1. Insufficient time to regain quiescent conditions in well after 
flowmeter movement. 

2. High ground currents (above- or below-ground power lines or 
power sources in vicinity of well or boreholes.) 

3. Coating on the electrodes such as mud or oil. 

4. Blockage of signal path to electrodes by gas bubbles. 

5. High flow rate entering well near location of flowmeter. 


III-4 Pumping Rate and Drawdown 
Measurements 

One of the most important aspects of the pumping test is the 
pumping rate, which should be high enough to achieve a 
measurable drawdown, if analyses of time/drawdown data are 
desired. The rate should also be low enough to support the 
assumption of negligible head loss within the well, if head loss 
across each interval is not measured. Additional 
considerations include methods for maintaining a constant 
flow rate throughout the test and constraints due to 
containment, treatment, and disposal requirements for 
contaminated water. In practice, it is often necessary to set the 
pumping rate by trial and error. After each trial start, 
sufficient time should be allowed for the well to recover 
before beginning a subsequent pumping test. 

Constant flow rates are important for two reasons. First, a 
constant flow rate permits calculation of horizontal inflow 
from aquifer intervals by subtracting the cumulative flow rates 
at different elevations. Second, a constant flow rate permits 
the use of conventional pumping test analytical solutions to 
calculate a transmissivity value at a well location. 


Problems associated with maintaining a constant flow rate are 
greatest where large drawdowns occur and the lift require¬ 
ments of a pump are affected. If no adjustments are made to 
the pump as the drawdown increases, the pumping rate will 
decrease until the drawdown stabilizes. In regions of low 
transmissivity, the problems associated with a constant flow 
rate can be severe because of large drawdowns. Although data 
analysis methods can be adjusted to compensate for changes in 
the pumping rate, these changes are not desirable. Injection 
tests may be considered instead of pumping tests if large 
drawdowns are expected. 

Accurate measurement of drawdown response during the 
pumping test generally requires a pressure transducer and data 
logger to record data at frequent intervals. As any movement 
in the well has the potential to alter the water level or the 
position of the transducer, sufficient drawdown data for 
calculating transmissivity should be collected before 
flowmeter testing begins. 

III-5 Selection of Pumps 

The type of pump used for the test depends on site conditions, 
well construction, and available power. If the depth of the 
water from the top of casing will not exceed approximately 7 
m, then a surface pump may be used. For flows greater than 4 
L/min, centrifugal surface pumps have proven to be reliable. 
This type of pump is tolerant of suspended solids and high or 
blocked discharge pressures, has good lift, and can provide an 
order-of-magnitude flow range by manually adjusting the back 
pressure. A disadvantage of this pump is the difficulty of 
presetting the pump to a known rate. A peristaltic pump is 
often a good choice for flows less than 4 L/min. It is easy to 
use, there is no contamination of the pump itself, and it may 
be preset within reasonable tolerances. 

Dissolved gases may be a problem with surface pumps when 
air bubbles accumulate in the intake line. The intake line 
pressure will be lowest near the surface pump. In some cases, 
the pressure may be sufficiently low to cause the water to 
degas. If sufficient gas accumulates in the intake line, then 
flow will be restricted to the pump and the pumping rate will 
decrease. In situations where degassing is a concern, the 
pump should be located above the well head to allow gas 
movement upward and through the pump. 

If the depth to water exceeds approximately 7 m, then 
submersible pumps are required. In general, submersible 
pumps present less operational problems than surface pumps. 
However, their use is often limited to wells with an ID greater 
than 5.1 cm due to the size of most pumps and the induced 
flow test design. If a submersible pump is used, it should be 
small enough to fit into the well and allow passage of 
flowmeter and pressure transducer cables. 

III-6 Packer Selection 

For wells greater than 5.1 cm in diameter, a packer may be 
needed to channel water through the flowmeter. If the 
velocities are sufficiently high, and the well has a constant 


14 





diameter, the flowmeter may be operated as a velocity meter 
without a packer. When used as a velocity meter, flow occurs 
around, as well as through, the EM flowmeter. Hence, a 
different set of calibration data is required in lieu of that 
obtained under conditions where packer assemblies are used. 
Where low flows are of concern and/or the well or borehole is 
not of constant diameter, a packer generally is required. 

Packer systems of different designs may be appropriate for 
these investigations. 

III-7 Method of Inflation 

Two methods generally were used for inflating/deflating the 
prototype inflatable packer assembly. One method 
incorporated an above-ground pressure chamber and the other 
used a submersible pump positioned above the flowmeter. 
Inflation using the pressure chamber was relatively slow and 
use was limited to sites where the water table was relatively 
shallow. The submersible pump has the advantage of rapid 
inflation and operates at wide ranges of depth to water and 
pressure heads. At Oak Ridge National Laboratory, Oak 
Ridge, Tennessee, submersible pump inflation has been 
successful at depths below 300 m, and depths-to-water of 
greater than 15 m. 

Installation using the above-ground pressure chamber was 
relatively simple. The pressure chamber has a fill port, a vent 
port, a pressure gauge, and a discharge port attached to a tube 
that inflates the packer. The most common setup involved 
filling the chamber 75 % full with water. The pressure 
chamber was charged to the required pressure with an air 
compressor or air tank. When the discharge valve was 
opened, the packer assembly inflated. Packer inflation was 
monitored by observing the decline in water level in the 
chamber. Adequate inflation was checked by tugging on a 
rope connected to the flowmeter. Packer deflation was usually 
accomplished by venting only the connection hose. 

In situations where the packer was inflated at large depths, or 
inflated and deflated relatively quickly, a submersible pump 
was used with the packer assembly. Several alternative and 
equally viable submersible pump configurations exist. A small 
ground-water sampling pump located approximately 0.3 m 
above the packer was used in these studies. The pump 
discharge was connected to the packer via a tube and integral 
solenoid valve. The assembly was wired such that the valve 
could not be closed when the pump was operating. In 
operation, the pump was turned on and the valve was opened 
to inflate the packer. Once the packer was inflated, the pump 
was stopped and the valve closed simultaneously. For 
deflation, the valve was opened, allowing the relatively high- 
pressure water inside the packer to flow back through the 
pump. Operation was reasonably fail safe as the pump could 
not operate against a closed valve, and the valve would open 
and deflate the packer upon power failure. 

Inflation times ranged from approximately 10 s to 15 s for a 
10.2-cm well to greater than 3 min for a 17.8-cm well. At 
shallow depths, packer inflation was checked by pulling the 


cable to determine if the flowmeter was firmly in place. 
However, at depths of 500 ft and greater the weight of the 
cable makes it difficult to check for inflation. At these depths, 
rapid increase in the electrical current drawn by the pump was 
monitored as an indicator of inflation. 

III-8 Issues Regarding Flowmeter 
Measurements in the Field 

Successful performance of investigations using the procedures 
described in this document requires knowledge of the 
mechanics of single-well pumping tests. This understanding 
provides the framework necessary to designate appropriate 
pumping rates, ascertain that flow conditions have stabilized, 
and select elevations for flowmeter measurements. In 
addition, familiarity with local geology, analytical solutions 
for pumping tests, and issues such as those listed below is 
necessary. 

a. Well or Borehole Storage 

During a pumping test, a common assumption is that the 
pumping rate equals the discharge from the aquifer. This 
assumption is acceptable only as long as a small percentage of 
discharge originates from within the well or borehole. In any 
large diameter well, borehole storage is a potential concern. If 
borehole storage dominates the response of a single-well 
pumping test, the time/drawdown data will appear as a straight 
line on a log-log plot with a slope of one. During a pumping 
test, the effects of borehole storage diminish with time and 
may be ignored at late times. Until storage effects dissipate, 
the drawdown will be less than if storage effects were not 
present. 

Two problems exist if flowmeter measurements are made 
before borehole storage dissipates. One problem is that the 
total flow into the well from the aquifer is increasing with 
time. Hence, the flow distribution to the well will be changing 
with time. Another problem is that calculated transmissivity 
values will be too high unless the drawdown values are 
corrected for wellbore storage. When performing a flowmeter 
test of this design, borehole storage should be considered in 
the analyses, especially in low permeability aquifers or in 
boreholes of large diameters. 

h. Selection of Pumping Rate 

Factors affected by the pumping rate include the magnitude of 
the total discharge, the drawdown response, and friction 
losses. In situations where ground-water contamination is of 
concern and discharged water may require treatment, a lower 
pumping rate is beneficial. Minimizing the drawdown also 
reduces the importance of vertical flow in unconfined aquifers. 
As discussed in Chapter I, horizontal flow is an assumption in 
the data analysis. For large drawdowns, horizontal ground- 
water flow is not a valid assumption for unconfined aquifers. 

During any flowmeter test, frictional losses occur through the 
well screen, well casing, and flowmeter. Because they are 


15 



difficult to measure in the field and can complicate the 
analysis of flowmeter data, these losses should be minimized. 
Conditions for which frictional losses may be of concern 
include heterogeneous highly transmissive aquifers and deep 
boreholes. In the former case, frictional losses associated with 
high flows entering the well through a narrow interval may be 
a problem if the total drawdown is small (e.g., < 25 cm) as the 
well loss becomes a significant fraction of the drawdown. In 
deep boreholes, cumulative friction losses associated with 
flow through a well pipe or borehole may be significant. 

Concerns associated with low pumping rates include: 
possible reduced sensitivity of the electromagnetic flowmeter, 
difficulty in monitoring the drawdown response, and 
adequately stressing the aquifer. Hence, although low 
pumping rates might be desirable, they should be high enough 
to ensure that the aquifer and well responses can be 
accurately measured with the available equipment. 

c. Background Electromagnetic Currents 

The flowmeter shell and telemetry cables are shielded 
against ground currents, but the inlet and outlet of the 
prototype flowmeter are not shielded. As a result, large 
background voltage gradients may influence the circuitry. 
Although the flowmeter is designed to reject such noise, its 
signal is in the micro-volt range and large voltage 
gradients may produce masking effects. Several wells 
where instabilities occurred in the flowmeter readings have 
been observed during development and use of the 
flowmeter. In such cases, the problems were believed to 
be caused by background currents. One of these wells, 
located near a building with large machinery, was surveyed 
twice. The first survey occurred with the machinery 
operating. During this survey, unstable flowmeter readings 
were recorded. The second survey occurred when the 
machinery was turned off. Stable, reproducible flowmeter 
readings were recorded. 

Due to the uncertainties associated with field testing, some 
simple laboratory tests were performed to better identify 
the problem. The tests involved inducing a voltage 
gradient across a pipe in the calibration facility and 
recording flowmeter responses. As long as the induced 
gradient was a sine wave synchronized with the power 
line, no instabilities were observed. However, once the 
voltage gradient varied in both amplitude and frequency, 
instabilities occurred. To shield the flowmeter from the 
voltage gradient, a metal screen was placed across the 
flowmeter's outlet and inlet. This modification reduced the 
sensitivity of the flowmeter's response, but appeared to 
eliminate the instabilities induced by background currents. 
In situations where electromagnetic background currents 
are large, problems may occur with the performance of the 
electromagnetic flowmeter. Preliminary investigations 
indicate these problems may be overcome by providing 
additional shielding for the flowmeter. However, 
additional testing is required before appropriate shielding 
can be developed. 


d. Reproducibility of Flowmeter Data 

A concern with any test is the reproducibility of the data. 
During a borehole flowmeter test, it is a good practice to 
repeat several flowmeter measurements without changing 
flowmeter position. If two measurements produce similar 
means and standard deviations, then the flowmeter system is 
assumed to be functioning properly. As standard practice, the 
flowmeter was considered to be performing adequately if two 
successive measurements produced mean values that 
overlapped in the range of their standard deviations. 

In developing a quality assurance plan for field testing, two 
types of duplicate measurements should be taken. The first 
type involves two successive measurements without moving 
the flowmeter. An assumption in comparing these values is 
that flow conditions in the aquifer have remained constant 
between measurements so that any differences reflect the 
uncertainties associated with the measurement technique. The 
second type involves measurements at the same location but at 
different times. Over periods of minutes and/or hours, the 
flow conditions in the aquifer may change. Differences in 
flow measurements would be primarily produced by variations 
in the saturated thickness of an unconfined aquifer in response 
to a pumping test, variations in the pumping rate caused by 
fluctuations in the performance of the pump, and inability to 
exactly reoccupy the same elevation in the well. The second 
type of duplicate measurement reflects the error associated 
with assuming that the flow conditions are at steady-state. 

III-9 Flowmeter Investigations in Consolidated 
Materials 

a. Introduction 

Many of the most intractable ground-water contamination 
problems occur in consolidated rocks and sediments where 
transport is dominated by flow through fractures or solution 
features. In systems where flow is dominated by secondary 
porosity (fractures, conduits, etc.), the identification of 
fractures that are hydraulically active and how they are 
interconnected is often important. This task may be 
complicated by the general observation that many fractures are 
inactive. Therefore, identifying fracture distributions using 
only a caliper log or a televiewer log provides limited 
information on the actual flow distribution that exists (Paillet 
et al., 1987; Paillet and Hess, 1987; Molz et al., 1990). 

Characterization of ground-water flow in fractured media will 
often involve measurements of flow to or from individual 
fractures or fracture zones. Over the past decade, researchers 
at the U.S. Geological Survey (USGS) have been refining 
procedures developed using a sensitive heat-pulse flowmeter 
(Hess, 1982, 1986; Paillet et al., 1987; Paillet and Hess, 1987; 
Hess and Paillet, 1990; Paillet and Kapucu, 1989; Paillet, 
1991a, 1991b, 1991c; Paillet et al., 1992). It is mainly their 
contributions that will be reviewed in the remainder of this 
section. The applications described below illustrate the need to 
incorporate a variety of geological, hydrogeological, and 


16 



geophysical tools in the investigation of site hydrology. 
Information from only one tool, such as a borehole flowmeter, 
will generally be insufficient to evaluate ground-water flow 
and contaminant transport and fate issues. This integrated 
characterization approach is necessary for investigations in 
porous media as well as fractured rock settings. 

b. Fractured Rock Applications 

Some of the most direct uses of a sensitive borehole flowmeter 
are in fractured rock investigations. These applications 
demonstrate the advantages in combining flowmeter 
information with other types of geophysical data. Perhaps the 
best way to describe this process is to present an example 
application in fractured dolomite in northeastern Illinois. This 
study was performed by the USGS (Molz et al., 1990). 

Acoustic-televiewer, caliper, single-point-resistance, and 
flowmeter logs were obtained in a 64-m deep borehole (DH- 


14) in the northeastern Illinois area as part of a study of 
ground-water flow and transport in fractured dolomite (Figure 
III-1). The acoustic-televiewer log is a magnetically 
orientated, television-like image of the borehole wall which is 
produced with a short-range sonar probe (Zemanek et al., 
1970). Irregularities in the borehole wall, such as fractures 
and vugular openings, absorb or scatter the incident acoustic 
energy, and result in dark features on the recorded image. 

Such televiewer logs may be used to determine the strike and 
dip of observed features. The acoustic-televiewer and caliper 
logs for borehole DH-14 (Figure III-l) indicate a number of 
nearly horizontal fractures that seem to be associated with 
bedding planes. The larger of these fractures are designated A, 
B, C, and D, respectively. The caliper log indicates that the 
major planar features on the televiewer log are large fractures 
or solution openings associated with substantial borehole 
diameter enlargements. The large but irregular features on the 
televiewer log between fractures B and C also are associated 
with borehole enlargements, but these are interpreted as 
vugular cavities within the dolomite rather than fractures. The 


Caliper Log 


Flowmeter Log 


ACOUSTIC 

TELEVIEWER LOG DIAMETER, IN INCHES 

5 6 7 8 9 10 



RELATIVE SINGLE-POINT 
RESISTANCE LOG 


DOWNFLOW, IN GALLONS 
PER MINUTE 



6.0 5.8 0.2 0.0 


INFLOW 


OUTFLOW 


INFLOW 


OUTFLOW 


30 


40 


50 


- 60 


iu 

o 

< 

li¬ 

ar 

3 

CO 


Q 

2 

< 


§ 

o 

—I 

UJ 

CQ 


CO 

a: 

LU 
i— 
LU 


I 

H 

a. 

LU 

a 


23.0 22.0 0.5 0.0 


DOWNFLOW, IN LITERS 
PER MINUTE 


Figure 111-1. Acoustic-televiewer, caliper, single-point resistance, and flowmeter logs for borehole DH-14 in northeastern Illinois (Paillet 
and Keys, 1984; Molz et al., 1990). 


17 























single-point-resistance log indicates abrupt shifts in resistance, 
at depths of about 40 m and 56 m. These shifts may reflect 
differences in the dissolved-solids concentration of the water 
in the borehole. 

The pattern of vertical flow determined by the flowmeter 
measurements indicates the probable origin for the inferred 
water quality contrasts in the borehole (Figure III-1). The 
flowmeter log indicated downflow, which probably was 
associated with naturally occurring hydraulic-head differences, 
causing water to enter at the uppermost fracture, A, and exit at 
fracture B. A much smaller flow of water with the same 
electrical conductivity and dissolved-solids concentration 
continued down the borehole to fracture C. At this fracture, 
the downflow increased and the water inflow apparently 
contained a greater concentration of dissolved solids, which 
accounts for the shift to greater electrical conductivity. This 
increased downflow exited the borehole at fracture D, where 
there was another, somewhat smaller, shift in single-point- 
resistance. Although not rigorously proven from the 
geophysical logs, the second shift in resistance appears to be 
associated with the dissolved-solids concentration of the water 
entering at fracture C. 

Subsequent water sampling confirmed that there were 
differences in the dissolved-solids concentration of the water 
at the different depths. Sample analysis indicated that the 
water entering at fracture A had a dissolved-solids 
concentration of about 750 mg/L; and the water entering at 
fracture C had a dissolved-solids concentration of about 1,800 
mg/L. In this instance, the geophysical data, especially the 
thermal-pulse flowmeter data, were useful in planning 
subsequent packer testing of the aquifer and in interpreting 
water quality measurements. Identification of substantial 
natural differences in background water quality was useful in 
conceptualization prior to modeling of conservative solute 
transport. At the same time, measurements of vertical velocity 
distributions in the borehole provided useful indications of 
hydraulic head differences between different depth intervals. 


This information could not be obtained from conventional 
water level measurements without the time-consuming 
installation of packers at multiple levels in all of the boreholes 
at the site. 

Paillet et al. (1992) consider the application of borehole 
flowmeters to measure flow transients in pumped boreholes 
and in adjacent observation boreholes. Their approach is 
based on the theoretical observation by Long et al. (1982) that 
fracture connections are more important than local fracture 
aperture in controlling the rate of flow through random 
distributions of finite fractures. Thus, their technique is 
designed to identify fracture connections. Paillet et al. (1992) 
also consider approaches to the problem at different scales of 
measurement which begins to attack the question of how 
individual fractures and fracture sets are integrated into larger- 
scale flow systems. 

c. Conclusions 

The previous study illustrates potential applications and 
integration of borehole flowmeters with other geophysical 
tools in the interpretation of flow in fractured aquifers. The 
relative ease and simplicity of flowmeter measurements 
permits reconnaissance of naturally occurring flows prior to 
hydraulic testing and identification of transient pumping 
effects. Flowmeter surveys may provide a valuable means by 
which to identify fracture interconnections and solute transport 
pathways during planning for much more time-consuming 
packer and solute studies. The borehole flowmeter is 
especially useful at sites where boreholes are intersected by 
permeable horizontal fractures or bedding planes. The simple 
and direct measurements of vertical flows provided 
information pertaining to the relative magnitude and vertical 
extent of naturally occurring hydraulic-head differences in a 
few hours of measurement. While the studies described in this 
section did not involve contaminated ground water, the 
potential application to contaminant migration problems and 
monitoring well screen location is obvious. 


18 



Chapter IV 

Well Construction and Development 


IV-1 Background 

a. In-Situ Hydraulic Conductivity Estimates 
Using Wells 

Well installation results in disturbance of aquifer materials. 
This immediately raises the question of what effects, if any, 
well construction and development techniques have on 
hydraulic conductivity measurements? There are many 
considerations that contribute to the answer of this question. 
Such considerations include the following. 

1. Is the subsurface material consolidated or 
unconsolidated? 

2. What is the relative importance of primary versus 
secondary porosity? 

3. What drilling technique is selected? 

4. Is the well constructed with an artificial filter pack? 

5. What drilling fluid, if any, is used? 

6. How is the finished well developed? 

7. What scale does the hydraulic conductivity 
measurement represent? 

It is beyond the scope of this report to provide a compre¬ 
hensive answer to the question of how well construction and 
development affects hydraulic conductivity measurements. 
Especially for small-scale measurements, this potential 
problem is neither well-documented nor understood. The 
approach herein will be to discuss the various drilling and 
development methodologies, review previous studies, discuss 
problems, and present test results concerning the sensitivity of 
borehole flowmeter measurements to well development and 
construction from two test sites. 

b. Formation Damages and Skin Effects 

All drilling methods alter the hydraulic characteristics of an 
aquifer near the wellbore. The impairment may be caused by 
the physical rearrangement of the matrix of aquifer material, 
by the smearing of silt and/or clay particles across the 
borehole face, or by invasion of drilling fluids or solids into 
aquifer formations. The amount of damage that occurs is 
related to the drilling method used for well construction and 
subsurface geology. The changes in hydraulic conductivity 
resulting from formation damage can produce skin effects in 
the near-well aquifer formations. 

The term “skin effect” was introduced by van Everdingen and 
Hurst (1949) when they discovered mismatches between 


analytical solutions and field data from well tests. The skin 
effect was caused by mud particle invasion into aquifer 
formations during well installation producing a decrease in 
hydraulic conductivity. Skin effects can be either negative or 
positive and represent increases or decreases, respectively, of 
hydraulic conductivity near the wellbore. Negative skin 
effects can be created at wellbores where drilling causes 
fractures in the aquifer and/or development causes excessive 
removal of fines from aquifer sediments. Negative skin 
effects may also result from well construction using an 
artificial filter pack that is significantly coarser than aquifer 
material. Positive skin effects can result from compaction, 
invasion of drilling fluids, smearing of clays, etc. 

Skin effects influence the flow distribution along the well. In 
single-well pressure transient tests, wellbore damage has been 
recognized (Dudgeon and Huyakorn, 1976; Chu et ah, 1980; 
Moench and Hsieh, 1985) as adversely affecting the tests. 
Analysis of the test data can result in large errors in estimates 
of storativity and transmissivity if skin effects and wellbore 
storage are not properly delineated. There are solutions 
available that account for both wellbore storage and 
infinitesimally thin skin in the pumping well (Sandal et ah, 
1978; Chu et ah, 1980) and both the pumping and observation 
wells (Tongpenyai and Raghaven, 1981; Ogbe, 1984; Ogbe 
and Brigham, 1984). The mathematical treatment used in 
these models to account for the skin region, however, is only 
an approximation and the hydraulic head drop across the skin 
is presumed to occur under steady flow conditions. 

Well development can reduce positive skin effects by repairing 
damage to the aquifer formations so that natural hydraulic 
properties are partially restored. For production wells, 
development comprises the systematic procedures followed to 
ensure the maximum discharge rate at the highest specific 
capacity with minimum production of particulate matter (Moss 
and Moss, 1990). More specific construction and 
development considerations are necessary for wells that are to 
be used to obtain geohydrologic data for aquifer 
characterization. 

IV-2 Well Design 

Ideally, wells designed for characterization using techniques 
described in this document should be fully screened across the 
interval of interest, such as the contaminant plume, be 
constructed without an artificial filter pack, and be screened 
such that the top of the screen is far enough below the water 
table to allow placement of the pump intake in the well casing 


19 



during the test. Boman et al. (1997) discuss the potential 
influence of an artificial filter pack on flowmeter results. A 
coarse-grained gravel or sand pack may result in significant 
vertical flow through the pack prior to entering the screen. 
This phenomena is enhanced by use of high flow rates that 
result in resistance to flow through the downhole probe of the 
electromagnetic flowmeter. In this situation, the measured 
flow distribution is skewed with a high influx of water near 
the top of the screen. For this reason, use of a borehole 
flowmeter in wells constructed with a filter pack of gravel¬ 
sized material may not provide meaningful data and generally 
should be avoided. It is also possible to observe some effects 
in wells with filter packs constructed of coarse-grained sand if 
the hydraulic conductivity of the pack material is significantly 
greater than aquifer materials. However, use of a natural 
collapse well construction is not always feasible. For 
example, formations with a significant fraction of fine-grained 
materials or units may not be appropriate for natural collapse 
construction and may result in void space within the annulus 
that affects test results. Results of field applications of the 
borehole flowmeter indicate that it is possible to obtain 
representative flowmeter measurements in many wells 
constructed using an artificial pack. In general, wells 
specifically constructed for these tests should avoid use of 
artificial filter packs, where possible. If an artificial pack is 
used, the data should be carefully examined for evidence of 
bias in the flow distribution. 

Wells that are screened across the water table may be subject 
to increased head loss near the pump intake. This is 
particularly true for wells constructed with an artificial filter 
pack that is significantly more conductive than surrounding 
aquifer materials (Boman et al., 1997). This situation may 
result in a nonuniform head distribution in the well and an 
increased influx of water near the pump intake. Such a 
situation would also bias flowmeter test results. 

Presence of a low permeability skin (i.e., positive skin effect) 
adjacent to the well may also significantly bias flowmeter 
results by affecting the distribution of flow to the well. 

Studies by Ruud and Kabala (1997) indicate that the presence 
of a low permeability skin adjacent to the well may have a 
much greater effect than the presence of a zone of increased 
permeability. As previously noted, the bias in the flowmeter 
measurements increases with increasing thickness of the 
disturbed zone and increasing difference between the 
permeability of the disturbed zone and the formation 
materials. 

These studies imply that wells should be designed and 
constructed to minimize the size of disturbed zones and the 
degree of disturbance. In general, formation of a higher 
permeability disturbed zone would be preferable to the 
presence of a lower permeability zone. The results also imply 
that the minimum size of measurement intervals deserves 
careful consideration in test design. Potential effects of well 
design and construction on the representativeness of the data 
obtained from these investigations should be carefully 
considered prior to well installation. Many of these questions 
are still areas for continuing research. 


IV-3 Well Installation Methods 

a. Overview 

Drilling methods chosen for construction of test wells should 
be those techniques that result in the least possible disturbance 
to the formation surrounding the well and result in a minimum 
annular space between the well screen and the borehole wall. 
The most appropriate method will depend on site-specific 
conditions. A number of references have appeared during the 
past decade regarding well drilling techniques, technologies, 
and related subject matter (e.g., Driscoll, 1986; Aller et al., 
1989; Harlan et al., 1989; and Moss and Moss, 1990). 

b. Drilling Techniques and Hydraulic 
Conductivity Estimates 

There are many possible interactions between the various 
drilling methods and the types of subsurface media at a 
particular site. Optimum methods depend on site conditions. 

In addition, the various studies that have dealt with the 
interaction of well drilling and hydrogeologic measurements 
have focused more on chemical measurements than on 
hydraulic measurements. Prominent studies during the past 
decade include Minning (1982), Barcelona and Helfrich 
(1986), Hackett (1987, 1988), Keely and Boateng (1987a,b), 
Paul et al. (1988), and Strauss et al. (1989). 

Keely and Boateng (1987a,b) present an illuminating 
discussion of various options and trade-offs when constructing 
monitoring wells. Much of the following discussion is based 
on those two references. Mud rotary has been a relatively 
common drilling technique for water supply wells because it is 
rapid, economical, and essential to the performance of certain 
geophysical logs. However, the technique has the 
disadvantage that the potential exists for large volumes of 
drilling fluids to enter the formation resulting in formation of a 
low permeability skin. Subsequent development sufficient to 
remove this skin effect is usually difficult or impossible. 

Improvements to this technology include driving a temporary 
casing while drilling or use of dual-wall reverse circulation 
drilling. In the dual-wall method, mud travels down a 
cylindrical annulus that is bounded by the outer drill pipe and 
an internal, rotating drill stem connected to the bit. Mud is 
ejected just above the bit, picks up cuttings, and is pumped up 
the inner drill stem to the surface. This technique limits 
exposure of the borehole wall to drilling fluids and decreases 
mud invasion into the formation. 

Many of the problems associated with introduction of drilling 
fluids using mud rotary techniques are also common to air 
rotary methods. These problems can be minimized by driving 
a temporary casing flush with the borehole wall. In fact, this 
is often necessary to prevent caving of the borehole walls 
when drilling in unconsolidated materials. However, 
compressed air can still enter the formation, and “air binding” 
is a well-known phenomenon that decreases the apparent 
hydraulic conductivity of porous materials. Just as with mud 


20 



invasion, air invasion would be expected to be particle-size 
dependent. 

According to Keely and Boateng (1987a), the cable tool 
method appears to have several advantages when used to 
construct monitoring wells, although it is rarely used for this 
purpose. In cable tool drilling, no drilling fluids are necessary. 
A temporary casing is driven as the drilling proceeds. Water 
may be added to the hole to aid bailing, but it is not under 
pressure. Driving and removing the temporary casing may not 
disturb the borehole wall significantly because there is a sharp 
drive shoe at the bottom, the casing is smooth, the annular 
spacing between the casing and the borehole wall is 
sufficiently small that material is not dragged along with the 
moving casing, and the casing is slowly moved up and down. 
This method may produce a well that is exceptionally well- 
suited for hydraulic conductivity and other measurements. 

The hollow-stem augering method is relatively fast, does not 
involve addition of drilling fluid, and formation samples may 
be obtained easily during the course of drilling. A major 
disadvantage, however, especially from the viewpoint of 
estimating hydraulic conductivity, is that the augering process 
causes wet clay and silt material to be smeared along the 
borehole wall. In certain types of soils such as clayey 
saprolites, the smearing and destruction of the open pore 
structure of the soil can be so severe that the resulting well is 
not suitable for hydraulic conductivity tests. Another potential 
disadvantage is that the annular space between a well screen 
installed through the hollow-stem auger and the borehole wall 
can be relatively large. Thus, if one does not wish to install a 
sand pack, there would be a relatively large volume into which 
the surrounding formation would collapse, further disrupting 
the structure of the natural formation. If collapse of cohesive 
soils was not complete, channels for vertical water movement 
would be formed. 

Much of the available literature contains information 
concerning well construction and measurements of various 
types in a variety of subsurface environments. This 
information is usually in the form of case histories, descrip¬ 
tions of problems that arose on particular projects, and the 
manner in which these problems were addressed. There have 
been relatively few investigations performed to study the 
effects of well construction on hydraulic conductivity and 
other measurements in a systematic fashion. One exception to 
this is the work of Morin et al. (1988b). In this study, 
epithermal neutron and natural gamma logs were used to 
measure the formation disturbance caused by three different 
well drilling techniques: hollow-stem augering; mud rotary; 
and hammer-driven, flush-jointed, temporary casing within 
which a permanent casing and screen were installed. Each 
type of construction was utilized for a number of wells so that 
a statistically significant set of logs could be obtained for each 
method. 

The study was conducted in a glacial outwash plane deposited 
in a braided stream environment. The study aquifer was 
composed of coarse sand and gravel in horizontal lenses and 
layers with silt and clay comprising less than five percent of 


the formation. This resulted in significant vertically- 
distributed heterogeneity that was reflected in the geophysical 
logs. The basic concept of the study was to associate an 
increase in formation disturbance due to well construction 
with a decrease in the degree of heterogeneity (more 
homogeneous appearance) observed in the well logs. Using 
this criterion, the order of increasing disturbance was: driven 
casing, mud rotary, and augering. 

IV-4 Well Development Methods 

a. Overview 

Well development includes a broad spectrum of techniques, 
procedures, and tools for applying some form of energy to the 
well screen and adjacent formation. Historically, several 
development methods that have been used following well 
installation include: overpumping, backwashing, mechanical 
surging, air development, and high-velocity jetting. Well 
development methods such as chemical treatment, hydraulic 
fracturing, and use of explosives to maximize the water yield 
in production wells are not applicable to these studies. 
Discussion in this text is restricted to aquifer characterization 
wells and development methods that assist in restoring the 
undisturbed hydraulic characteristics of aquifer formations. 

Determining which development methods are most 
appropriate for any given well requires an understanding of 
available methods, a knowledge of the aquifer formations 
present at the well location, and particulars of the well design. 
In some cases, a single method might be effective for 
development. However, in many instances, the use of more 
than one development method will yield much better results. 
Driscoll (1986), Moss and Moss (1990), and Harlan et al. 
(1989) explain the mechanics of the different well 
development methods. 

b. Development Methods and Hydraulic 
Conductivity Estimates 

The literature offers little guidance on the selection of an 
optimum method of well development for any particular site. 
Much of the former work related to well development has 
concentrated on maximizing water yield in production wells. 

In general, these types of studies (National Ground Water 
Association, 1989) provide evaluations of different well 
drilling and development techniques based upon specific 
capacity measurements. This is a measure of the reduction in 
positive skin effects produced by well development. 

Although the influence of skin effects on pumping tests has 
long been recognized in the petroleum industry (e.g., van 
Everdingen and Hurst, 1949; Hawkins, 1956) and by ground- 
water scientists (e.g., Moench and Hsieh, 1985), very few 
studies provide comparisons of different development methods 
using explicit hydrogeologic measurement techniques. 

Rehfeldt et al. (1989b) provide results from experiments that 
were undertaken using an impeller flowmeter to investigate 
well installation and development methods on flowmeter 


21 



discharge profiles. The test site was a heterogeneous sand and 
gravel aquifer located at the MacroDispersion Experiment 
(MADE) site on the Columbus Air Force Base, Columbus, 
Mississippi. One of the major objectives of the study was to 
determine the most appropriate well installation and 
development method that would minimally disturb the aquifer 
and allow accurate estimates of hydraulic conductivity at the 
test site. Wells evaluated in the experiment included a driven 
steel well screen, drive-and-wash wells using compressed air, 
and hollow-stem auger wells, constructed with both natural 
and artificial filter packs. The wells were developed in three 
stages; first by cyclic overpumping and backwashing, then 
with a surge block attached to the drill rig, and finally using a 
hand-drawn swab. Hydraulic conductivity profiles were 
determined from impeller flowmeter measurements after each 
stage of development. When hydraulic conductivity estimates 
were reproducible, well development was considered 
complete. 

The study indicated that a large diameter (30 cm) hollow-stem 
auger method of well installation was the least satisfactory of 
the test methods because of the degree of disturbance 
associated with auger drilling and the fact that more annular 
space exists between the wellbore and screen. Although the 
driven steel well screen was installed to represent a well with 
no open annular space, it was substantially more expensive 
than other methods because of material costs. The internal 
vertical rods that support the steel screen also presented 
problems with well development since the surge block and 


swabbing tool could not develop a good seal along the inside 
of the screen. Rehfeldt et al. (1989b) provide the following 
observations based on their testing. 

1. Well development caused changes in the aquifer 
material adjacent to the wellbore and altered the 
hydraulic conductivity profiles. 

2. Overpumping and backwashing development 
produced changes in the profiles of both the augered 
well and the well installed using drive-and-wash 
methods. Order-of-magnitude increases in hydraulic 
conductivity were observed at a minimum of five 
locations for the augered well, but were never that 
large for the drive-and-wash well. Additionally, the 
trends in the profiles after overpumping and 
backwashing development were generally similar in 
the drive-and-wash well but not in the augered well. 

3. After swabbing, the augered well again showed 
significant differences in the profiles, although less 
than for the first two development cycles. The profile 
for the drive-and-wash well after swabbing was 
relatively consistent. 

4. The profile for the driven steel screen generally had 
lower values than the profile for both the augered and 
drive-and-wash wells. The lower hydraulic 
conductivity values may have been partly caused by 
compressed aquifer material near the well, the 
adherence of fine-grained material to the well screen, 
or incomplete development of the well. 


22 



Chapter V 

Field Studies of Well Construction and 
Development at Columbus, Mississippi 


V-l Description of Test Site 

a. Site Location 

The test site occupies one of twenty-five hectares (Ha) in the 
northeastern corner of Columbus Air Force Base (CAFB), 
Columbus, Mississippi, leased from the U.S. Air Force for the 
MacroDispersion Experiment (MADE). The site is 
approximately 6 km east of the Tombigbee River and 2.5 km 
south of the Buttahatchee River, and lies above the 100-yr 
floodplain of both rivers. The site is situated on the youngest 
terrace deposits associated with the Buttahatchee River. 

b. Aquifer Characteristics 

Young (1991b) describes the Columbus Aquifer as being 
composed of approximately 10 m of Pleistocene and Holocene 
fluvial deposits. The aquifer overlies the Eutaw formation that 
consists primarily of marine clay and serves as an aquitard. In 
addition to numerous samples from boreholes, a geological 
investigation of the aquifer included mapped geological facies 
at a gravel pit and an aerial photography survey that indicated 
an abandoned river meander crossing the northern region of 
the 1-Ha test site and passing through the middle of the 
MADE test site (Figure V-l). 

The aquifer at the site is composed of poorly-sorted to well- 
sorted sandy gravel and gravelly sand with minor amounts of 
silt and clay. Sediments are generally unconsolidated and 
cohesionless below the water table. The upper portion of the 
aquifer is generally composed of coarse-grained sediments and 
bar deposits from a meandering river system (Kaye, 1955; 
Rehfeldt et al., 1989b). The abrupt changes in vertical 
sequences and coarse texture suggest that the pointbar 
sediments were deposited during catastrophic flooding events. 
Chute bars and channels with clay drapes were also deposited 
during flood stage. Such events result in an uneven sand 
distribution and a seemingly chaotic occurrence of gravel 
lenses and clay drapes (Collinson and Thompson, 1989). 

The lower portion of the aquifer is composed of sediments 
from a braided river system. This model implies an irregular 
pattern of coarse gravelly lenses deposited as longitudinal and 
transverse bars at high flow stage, alternating laterally and 
vertically with finer sand and silt deposited in channels at low 
flow stage. As these depositional bodies were formed by 
short-lived branching and rejoining shallow channels, and 
subsequent partial truncation by secondary stream channels, 



Figure V-1. Ox bow meander at the Columbus AFB site drawn 
from a 1956 aerial photograph. 

a large range of shapes and sizes exist for the depositional 
bodies. The shapes might best be described as irregular 
tongues, shoestrings, wedges, and pods. 

c. Previous Pumping Tests 

A series of single-well pumping tests, slug tests, and 
electromagnetic borehole flowmeter tests were conducted at 
the original thirty-seven wells located across the 1-Ha test site. 
The results of these tests indicate that a zone of interconnected 
high-K deposits exists at elevations of 59 m to 62 m above 
mean sea level (AMSL) within boundaries mapped by the 
former meander shown in Figure V-1. These results also 
indicated that positive (low-K) skin effects exist at most, if not 
all, of the wells. Young (1991a,b) provides an analysis of the 
pumping test data using the Cooper-Jacob equation and the 
Cooper-Jacob Straight-Line (CJSL) method (Cooper and 
Jacob, 1946). The analysis indicates that the two approaches 
provide significantly different transmissivity values. 

Considering that poorly-sorted coarse-grain sediments often 
lie adjacent to well-sorted fine-grain sediments, and that 
highly permeable lenses can dominate ground-water flow at 
Columbus, significant skin effects were considered probable. 
The following are potential causes of positive skin effects: 


23 





















(1) smearing of silt and clay particles into and across high- 
permeability zones; (2) compaction of aquifer material by 
methods that include advancement of a protective casing; and 
(3) alignment of blank (i.e., nonslotted) sections of well casing 
with aquifer zones of high permeability. 

d. Monitoring Well Installation 

For the purposes of this study, two clusters of three wells each 
were added to an existing network of thirty-seven wells at the 
1-Ha test site (Figure V-2). The drilling and development 
methods used to install the first thirty-seven wells at the 1-Ha 
test site are described by Young (1991a). The six new wells 
are numbered 38 through 43. The well clusters were 
configured as equilateral triangles with individual wells being 
separated by distances of two meters so that aquifer 
conditions near each well would be similar. The two well 
clusters are separated by a distance of about 60 m. Well 
cluster 38-39-40 resides on the southern side of the 1-Ha test 
site within possible pointbar deposits in the upper aquifer and 
possible braided river deposits in the lower aquifer. Well 
cluster 41-42-43 is located in the northern portion of the test 
site, within possible channel sediments from a meandering 
river system and with possible channel sediments at depth 
from a braided river system. 

Each well is approximately 11 m deep and consists of 5.1-cm 
(inside diameter), schedule-40 PVC casing and screen (Figure 
V-3). The screen for each well is 9.14 m in length and 
machine-slotted with 0.25-mm slots spaced at 3.18-mm 


intervals. The following three drilling methods were used for 
wells at each well cluster: 

1. 19.4-cm hollow-stem auger - natural fdter pack 

2. 27.0-cm hollow-stem auger - artificial filter pack 

3. 11,4-cm drive-and-wash - natural filter pack 

Wells 39 and 43 were installed using a 19.4-cm (outside 
diameter) hollow-stem auger. The aquifer material was 
allowed to collapse around the screen and casing, and auger 
cuttings were used to backfill approximately the top two 
meters of borehole. Wells 38 and 41 were installed using a 27- 
cm (outside diameter) hollow-stem auger with artificial filter 
packs. The filter pack was comprised of 1.6 mm to 3.2 mm, 
subangular to angular, quartz sand. A disadvantage of the 
larger auger is the size of the annulus created. 

Wells 40 and 42 were installed using a modified rotary wash 
method. The method involved driving an 11.4-cm steel outer¬ 
casing ahead of the rotary bit and then washing the cuttings 
out of the casing with water. The well screen and casing were 
lowered into place, and the outer steel casing was removed 
allowing formation material to collapse around the well. The 
modification minimizes both the wash into undisturbed aquifer 
sediments and the amount of formation material removed. 

In unconsolidated aquifer materials, the driving of casing 
generally creates a region of compacted aquifer material 
around the casing. Pulling the outside casing and allowing 
aquifer material to collapse on the inner well screen and 
casing may relieve this compression. Based upon studies by 



Figure V-2. Well network at the 1-Ha test site. 


24 









D=D 



Borehole by 
15.9-cm ID, 
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Auger 

5.1-cm 
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(a) 

WELLS 38 &. 41 
ARTIFICIAL FILTER PACK 



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(c) (NOT TO SCALE) 

WELLS 40 & 42 
ROTARY WASH 


Figure V-3. Design of wells used to evaluate the effect of well development on flowmeter tests. 


Rehfeldt et al. (1989a) at CAFB, this was expected to be the 
preferred method of installation as the extent of aquifer 
disturbance is much less than in auger drilling. The annular 
space surrounding the well into which the aquifer material 
must collapse is also effectively reduced from about 6.7 cm 
for small (19.4-cm) diameter auger wells to 2.7 cm for the 
driven wells. Hence, the in-situ hydraulic conductivity 
estimated from a well installed by drive-and-wash methods is 
more likely to be representative of the true aquifer hydraulic 
conductivity. 

V-2 Test Descriptions 

This study consisted of a series of single-well pumping tests 
conducted at six wells. Between pumping tests, additional 
well development was performed. During each pumping test, 
drawdown and electromagnetic flowmeter data were collected. 
Testing at each of the two well clusters was conducted 
concurrently using two teams outfitted with flowmeters, 
pressure transducers, data loggers, lap-top computers, and 
other equipment required for monitoring and testing. The 
flowmeters were calibrated against known flows prior to, and 
after, their use in the field. Both 1.27-cm and 2.54-cm (orifice 
diameters) electromagnetic flowmeters were used during 
testing. 

The procedure for sequential development and testing ot the 
monitoring wells consisted of the following steps: 


1. Ambient flowmeter tests were conducted at each 
well. 

2. Pumping tests and flowmeter tests were 
performed. 

3. Wells were developed according to prescribed 
methods. 

4. Retesting and development sequence was completed 
for three development cycles. 

5. Injection tests and high-rate pumping tests were 
performed. 

Ambient flowmeter tests were conducted to measure natural 
(background) ground-water flow within each well. The type 
of well installed, the aquifer formations present, and the 
degree of formation damage from drilling have differing 
effects on ambient flows. Ambient measurements were taken 
at 30.5-cm increments beginning at the bottom of the well and 
advancing upwards until the water table was reached. The 
ambient measurements were taken after the test wells had 
sufficient time to return to a quiescent state from development 
activities and pumping tests, which was usually overnight. 

Single-well pumping tests and borehole flowmeter tests were 
completed at each well following ambient flowmeter testing. 
Discharge measurements were made several times during each 
test. The flowmeters were used to obtain a profile of the flow 
distribution after a steady flow field was achieved in the 
vicinity of the well, which generally occurred after 20 min to 


25 



























































30 min of pumping. Flowmeter readings were obtained at 
30.5-cm increments as in ambient testing. Flow distribution 
profiles were compared in the field after progressive well 
development. Test wells were allowed to fully recover before 
proceeding with each stage of development, usually overnight. 

A total of six pumping and flowmeter tests were completed for 
each well (Table V-l). The first four tests were conducted 
prior to and after each development cycle. The last two tests 
were injection and high-rate pumping tests. The first method 
used for well development was overpumping and 
backwashing. Initially, the well was pumped until the water 
began to clear. Development was then completed by pumping 
water through a discharge hose several feet above the top of 
the well, and then allowing it to flow back through the pump 
and out through the well screen. Water was occasionally 
discharged to remove the fine-grained materials. Several 
pumping cycles were completed until the discharge was clear. 

The second development consisted of a modified swabbing 
method that was alternated with overpumping and 
backwashing. This method is similar to the procedure used 
by Rehfeldt et al. (1989b). A section of galvanized pipe was 
added to weight the swab and increase its rate of fall. The 
swab and galvanized pipe had a combined weight of over 2.9 
kg. Beginning at the bottom of the well, the swab was 
manually raised with force over a 1-meter increment of the 
well screen and then allowed to fall back to its original 
position to provide a surging action. This swabbing 
movement was repeated for a total of four repetitions, until 
the water table was reached. Water was then pumped to 
remove fines liberated during development and overpumping/ 
backwashing was conducted. The entire cycle was repeated 


using three swabbing repetitions. The third and final 
development was a reiteration of the alternating modified 
swabbing and overpump/backwash method that was identical 
to that used in the second development. 

J 

V-3 Test Analyses and Results 

a. Ambient Flow Distributions 

Ambient flow measurements collected after each successive 
phase of well development (Figure V-4) varied from -0.34 L/ 
min to 0.04 L/min for well cluster 38-39-40, and from -0.06 L/ 
min to 0.29 L/min for well cluster 41-42-43. The sign 
convention is positive for upward flow and negative for 
downward flow. The initial development, if properly 
conducted, should displace extraneous fines from the 
wellbore. From observation of the flowmeter plots, it is 
apparent that the initial development (overpumping and 
backwashing in this case) is important. A relatively large 
change in ambient flow is displayed after the first 
development by all wells except wells 42 and 43. The 
magnitude of the changes in ambient flows decrease with 
ensuing stages of development. Following the first stage of 
development, the changes that occur in the ambient flows also 
vary according to well type and differences in aquifer 
materials. The modified rotary wash wells (40 and 42) exhibit 
ambient flow profiles that were essentially reproducible after 
the first stage of development. 

The ambient flow profiles for the wells constructed with an 
artificial filter pack (38 and 41) show differing responses after 
the second and third development (Figure V-4). The ambient 


Table V-1. Testing Sequence 

Test 


No. 

Test Type 

Development Cycle 

Development Method 

1 

Ambient Borehole Flowmeter (BHFM) Tests 
Constant Discharge Pumping Tests 

BHFM Tests During Pumping 

Pre-development 

None 

2 

Ambient BHFM Tests 

Constant Discharge Pumping Tests 

BHFM Tests During Pumping 

After 1st development 

Overpump/Backwash (15 minutes) 

3 

Ambient BHFM Tests 

Constant Discharge Pumping Tests 

BHFM Tests During Pumping 

After 2nd development 

Alternating Modified Swabbing and 
Overpump/Backwash (60 minutes) 

4 

Ambient BHFM Tests 

Constant Discharge Pumping Tests 

BHFM Tests During Pumping 

After 3rd development 

Alternating Modified Swabbing and 
Overpump/Backwash (60 minutes) 

5 

Ambient BHFM Tests 

Constant-Rate Injection Tests 

BHFM Tests During Injection 

Development complete 

None 

6 

Ambient BHFM Tests 

Constant Discharge Pumping Tests 

BHFM Tests During High-Rate Pumping 

Development complete 

None 


26 





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flows at well 41 increase incrementally with diminishing 
differences between development cycles. Well 38, on the 
other hand, exhibits some irregularities in ambient flow 
profiles. Below about 56 m, the first development of well 38 
produces the highest ambient flows with succeeding 
development resulting in reductions of ambient flow. Above 
about 56 m, the second development of well 38 produces the 
highest ambient flows with ambient flows being reduced 
following the third development. These responses may simply 
be due to stabilization of the filter pack or to effects of minor 
temporal changes in local hydrology. 

The augered wells with natural filter packs (39 and 43) display 
ambient flow profiles that are quite variable (Figure V-4). For 
both wells, the ambient flows measured following the second 
stage of development are greater than those of the third stage of 
development. The two wells also show the largest differences in 
their ambient profiles between the second and third development 
cycles. These results suggest that the material around the 
augered wells was the most sensitive to well development. 

Several shifts and peaks in the ambient flow profiles were 
observed through the first two stages of well development for 
both well clusters. These phenomena are attributed to 
inadequate collapse and stabilization of aquifer material 
around the screens for the auger hole wells, and to incomplete 
grading and stabilization of aquifer and filter pack materials 
for the wells with artificial filter packs. Very little deviation in 
the ambient flow profiles are shown by the modified rotary 
wash wells after initial development. By the third stage of 
development, the shifts and peaks in all the ambient flow 
profiles had decreased to the point that wellbore material was 
stabilized and development was deemed complete. 

Well cluster 38-39-40 shows good correlation between wells 
for the final profiles of ambient flow. The profiles indicate 
that most of the water is entering between about 57 m and 58 
m AMSL and leaving between 54 m and 55 m AMSL, with 
little ambient flow above 58 m AMSL. These two high 
ambient flow zones suggest that sequences of high hydraulic 
conductivity sediments may intersect the wells at these 
horizons. For well cluster 41-42-43, there is less similarity 
among the ambient flow plots. All three wells have upward 
ambient flow but there is considerable variability in the source 
and magnitude of the flow. 

b. Induced Flow Distributions 

The flow distributions measured under pumping conditions 
(Figure V-5) represent the results from flowmeter testing during 
steady-state flow conditions. The mean flow rates used for 
pumping tests were 8.0 L/min and 13.4 L/min for well clusters 
38-39-40 and 41-42-43, respectively. The pumping rates used 
for the tests were fairly constant and had variances of only 0.015 
L/min and 0.076 L/min for well clusters 38-39-40 and 41-42-43, 
respectively. The cumulative flows were adjusted by taking the 
net difference between the measured flows under pumping and 
ambient conditions, and then normalizing the result to the 
pumping rate. 


The induced flow distributions for wells 41 and 43 show 
changes after the initial development that are relatively large 
in comparison to succeeding development cycles. The 
magnitude of the changes in flow decreases incrementally 
with ensuing stages of development for the well with the 
artificial filter pack (well 41). Observation of the induced 
flow distributions for well cluster 38-39-40 does not indicate a 
large change in the profiles of wells 38 and 40 between pre- 
and initial development. The greatest change in the flow 
profiles for these two wells occurs after the second 
development, indicating that the modified swabbing method 
was necessary to effectively develop the wells. The induced 
flow profiles for well 39 indicate that it is experiencing a 
moderate degree of formation stabilization during later stages 
of development. 

c. Specific Capacity Values 

Drawdown data (Figure V-6) were used to calculate specific 
capacity (Figure V-7) for each well based on the drawdown 
responses at 1500 s after initiation of extraction. In general, 
the greatest increases in specific capacity occur after the initial 
development. Except for well 41, specific capacity values 
stabilized after the second development. Well 41 has an 
artificial filter pack and is located in a region where highly 
transmissive aquifer zones exist near the top of the well 
screen. The different response at well 41 may be caused by 
spatial variability within the aquifer and the possible 
intersection of a highly permeable lens that is not 
interconnected with the neighboring wells. 

d. Transmissivity Values 

The single-well test drawdown curves (Figure V-6) were 
analyzed using the Cooper-Jacob straight line (CJSL) method 
(Cooper and Jacob, 1946) to calculate transmissivity. The 
semilog plots of drawdown data are characterized by an early 
slope that is about five to twenty-five times steeper than the 
late slope (after 100 s). For comparison purposes, 
transmissivities were calculated for both early and late times. 
Typically, the late-time portion of the curve used for the CJSL 
analysis began between 100 s and 300 s and ended between 
900 s and 1,000 s. Late-time average transmissivities were 
calculated using the data from all four pumping tests at each 
cluster. The average transmissivities for well clusters 38-39- 
40 and 41-42-43 are 19.8 cm 2 /s and 34.0 cm 2 /s, respectively. 

At every well, the ratio of the transmissivity after each well 
development to the average transmissivity were within a factor 
of two. The results indicate increases and decreases in 
transmissivity with additional well development. However, no 
consistent trends are evident in the data. It is possible that the 
changes in the calculated transmissivity values were caused 
more by the differences in the testing and analyses procedures 
than the aquifer conditions. In order to minimize recovery 
time between pumping tests, low pumping rates were used. 

As a result, trends in the drawdown curves are difficult to 
define because the aquifer was not adequately stressed. 

The CJSL analysis for the early-time portion of the semilog 
drawdown curves typically began near 20 s and ended at about 


28 



WELL 38 - ARTIFICIAL FILTER PACK WELL 39 - AUGER HOLE WELL 40 - ROTARY WASH 






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TIME (seconds) TIME (seconds) 


PUMPED WELL 
WELL 38 


OBSERVATION WELL 
WELL 39 WELL 40 


WELL 39 - AUGER HOLE 



AFTER FIRST 
DEVELOPMENT 


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TIME (seconds) 



PUMPED WELL 

OBSERVATION WELL 

WELL 39 

WELL 38 WELL 40 

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WELL 40 - ROTARY WASH 




1 2 1020 100200 1,000 1 2 1020 100 200 1,000 1 2 1020 100 200 1,000 
TIME (seconds) TIME (seconds) TIME (seconds) 


PUMPED WELL 

OBSERVATION WELL 

WELL 40 

WELL 38 WELL 39 




Figure V-6. Effect of well development on the drawdown values for pumping tests at the 38-39-40 well cluster. 


30 



























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Figure V-7. Comparison of specific capacities at different wells. 


80 s. Values of early-time transmissivity (Figure V-8, Table 
V-2) only increased with successive well development. These 
increases are interpreted as reductions in positive skin effects. 
There are no similarities evident for early-time transmissivity 
increases among wells of a particular construction type. 
However, well cluster 38-39-40 exhibits increases in early- 
time transmissivity values that are up to an order-of-magnitude 
less than estimates for well cluster 41-42-43. This appears to 
be a reflection of the low transmissivity region in which 
cluster 38-39-40 resides. The geometric means for early-time 
transmissivity values were 1.6 cm 2 /s and 3.2 cm 2 /s for well 
clusters 38-39-40 and 41-42-43, respectively. These averages 
are approximately ten times lower than the transmissivity 
values measured at late times, and are in good agreement with 
that of 3.6 cm 2 /s calculated by Young (1991a,b) for early-time 
slopes from fifty-seven single-well pumping tests and thirty- 
one slug tests at the 1-Ha test site. 

V-4. Summary 

A series of flowmeter tests were conducted at Columbus Air 
Force Base to investigate the effects of well construction and 
development methods on flow distributions and drawdown 
responses in a highly heterogeneous aquifer. Two well 
clusters were installed 60 m apart. Each well cluster included 
three 5.1-cm diameter PVC wells installed using different 
drilling techniques. The three installation techniques included 
a modified rotary wash method, a 19.4-cm hollow-stem auger 
method, and a 27.0-cm hollow-stem auger method using an 


artificial filter pack in well construction. Differences in local- 
scale geohydrology are immediately apparent from 
observation of the ambient flow profiles. At one well cluster, 
all three wells exhibit only downward ambient flow. At the 
other well cluster, upward ambient flow was measured. 

The test results show that well development has a significant 
impact on increasing the magnitude of ambient flows. At 
several vertical zones, the directions of ambient flow changed 
because of well development. The artificial filter packed and 
rotary wash wells displayed the greatest changes in ambient 
flow profiles following initial well development by 
overpumping and backwashing. The largest changes for the 
augered wells with a natural filter pack occurred after the 
second well development, which included swabbing with a 
surge block accompanied by overpumping and backwashing. 

Among the ambient flow profiles, those from the rotary wash 
wells had the least sensitivity to well development beyond the 
initial development. At the two rotary wash wells, the ambient 
flow profiles stabilized after the initial well development. At 
the “19.4-cm augered” wells and one of the augered wells with 
an artificial filter pack, the ambient profile patterns remained 
similar after the initial well development, but the magnitude of 
the flow rates differed. The data clearly indicate that some 
type of well development is necessary to obtain useful 
flowmeter data for ambient conditions, and suggests that the 
amount and method of well development depends on the well 
type and aquifer properties. 


31 





























2.2 


- 0 ? 

c\T 

E 

(A 

Q) 

_3 

CC 

> 

>> 

> 

CO 

CO 

E 
<o 
c 
03 


CD 

E 


03 

LLI 


2 - 
1.8 - 
1.6 - 
1.4 
1.2 


1 

10 

8 

6 

4 

2 

0 


A 

.0 


A 

O 


■ Well 38-Artificial 

Filter Pack 
▲ Well 39-Auger Hole 
• Well 40-Rotary Wash 


o 

A 


O 

A 


O Well 41-Artificial Filter Pack 
A Well 42-Rotary Wash 
o Well 43-Auger Hole 


Pumping Test Number 


Figure V-8. Calculated transmissivities using early time data. 


Table V-2. Early-Time CJSL Transmissivity Values 


Well No. 

Initial T 
(cm 2 /s) 

Final T 
(cm 2 /s) 

Percent 

Increase 

38 

1.2 

1.7 

42 

39 

1.7 

2.1 

25 

40 

1.6 

1.9 

18 

41 

3.8 

8.8 

129 

42 

1.4 

4.9 

242 

43 

0.9 

5.6 

494 


The flowmeter results for pumping conditions produced 
conclusions similar to those obtained for the ambient flow 
conditions. However, two differences were observed. The 
first difference was a convergence to a stabilized flow profile 
for all of the well types. The second was the effect of the well 
cluster locations on the relative differences among the flow 
profiles. One well cluster displayed the greatest flow profile 
changes between the “pre-development” and “1st 
development.” The other well cluster showed the greatest 
differences between profiles for “1st development” and the 
“2nd development.” 


within the well must temporarily exit the well. This might 
occur near regions where voids existed in the well annulus 
and/or the flowmeter caused significant blockage of incoming 
flow. The flow bypass problem can be lessened by creating a 
more uniform packing in the well annulus. In subsequent tests 
after well development, the frequency and magnitude of the 
flow bypass problems were greatly reduced. 

For each flowmeter test, the drawdown data in the pumped 
well were analyzed using the CJSL method to estimate 
transmissivity. Typically, the drawdown response was 
characterized by a linear slope at early times (<50 s) that was 
about five to twenty-five times steeper than the late slope 
(after 100 s). The early slope values are reflective of the 
disturbed aquifer material near the well. The late-time values 
better represent undisturbed aquifer material at radial distances 
further away from the well. The CJSL analysis shows that the 
transmissivity values calculated at early times only increased 
with additional well development. Although positive skin 
effects were not eliminated, these results indicate that the 
effects were significantly reduced. 


At several elevations in the “pre-development” profiles, a 
decrease in net differential flow occurred with increasing 
elevation. For this decrease to occur, vertical flow established 


32 
























Chapter VI 

Field Studies of Well Construction and 
Development at Mobile, Alabama 


VI-1 Description of Test Site 

a. Site Location 

The well field is located at the Barry Steam Plant, which is 
owned and operated by the Alabama Power Company. 
Geographically, it is approximately 32 km north of Mobile, 
Alabama. 

b. Aquifer Characteristics 

The surface is composed of a low terrace deposit of 
Quaternary age consisting of interbedded sands and clays that 
have, in geologic time, been recently deposited along the 
western edge of the Mobile River. The sand and clay deposits 
(Figure VI-1) extend to a depth of 61 m where the contact 
between the Quaternary and Tertiary formations is located. 
Below the contact are deposits of the Miocene series that 
consist of undifferentiated sands, silty clays, and thin-bedded 
limestones extending to an approximate depth of 305 m. 


The shallow subsurface consists of fluvial sediments with a 
confined aquifer in the bottom 20 m of the Quaternary 
sediments. The aquifer is confined above and below by clay 
bearing strata that extend laterally for several thousands of 
meters or more. The upper confining layer is located about 
40 m deep, and the thickness of the aquifer is relatively 
constant at about 20 m. The piezometric surface for the 
confined aquifer ranges from land surface elevation to two or 
three meters below land surface depending on seasonal and 
climatic conditions. In general, the confined aquifer matrix may 
be described as a medium sand with silt and clay fractions 
ranging from one percent to fifteen percent by weight. 

c. Well Installation 

Four observation wells at the Mobile test site were designed to 
provide two distinct environments for construction and 
development studies. Two of the wells (Figure VI-2) were 
deep wells and were installed to depths of 60 m, with fully- 
penetrating screens in the confined zone between 40 m and 
60 m. The other two wells were shallow wells drilled in the 



Figure VI-1. Vertical cross-sectional illustration of the subsurface hydrologic system at the Mobile site. 


33 



































Figure VI-2. 


Schematic diagram providing the details of the shallow and deep wells constructed at the Mobile site. Well casing and well 
screen are the same dimension and schedule. The only difference is the depth dimensions of the casing and screen. 


sand, gravel, and clay strata (Figure VI-1) to a depth of 30 m 
and screened between 4.5 m and 30 m. The layout of the 
wells is in a 10 m xlO m square arrangement with a well 
positioned at each corner. The designations for the wells are A 
and B for the shallow wells, and C and D for the deep wells. 
During construction, a portion of the installed casing of well D 
was fractured allowing sediment to enter the well. Thus, well 
D was then considered non-functional and removed from the 
study. All wells were installed using the direct circulation 
rotary drilling method. The shallow wells were drilled to 
accommodate a 15.2-cm schedule-40 PVC casing from ground 
surface to an approximate depth of 4.5 meters. The casing 
was sealed in place by cement grout and left undisturbed for 
12 hours. Following the 12-hour period, the well was drilled 
to its final depth (30 m) to receive a 10.2-cm slotted schedule- 
40 PVC screen. No artificial filter pack material was used in 
the construction of these wells. A 3-m section of 10.2-cm 
PVC pipe was attached to the screen and extended upward 
approximately 3 m from the bottom of the casing. The annular 
space between the pipe and the 15.2-cm casing was closed by 
a packer. The deep well was constructed in the same manner 
using the same size casing and screen. After completion, each 
well was developed by forcing air into the casing to agitate, 
surge, and lift the water; mildly cleaning the well of drilling 
fluid, cuttings, and fines. 


VI-2 Test Description 

The direct rotary drilling method used a mud slurry as a 
drilling fluid during the construction operation. Site 
conditions necessitated use of this method. However, the mud 
cake has a deleterious effect on the production capacity of the 
well due to its very low hydraulic conductivity. Development 
was necessary to remove the mud cake to the extent practical. 
In this study, air development was used to surge compressed 
air into the casing of each well for approximately fifteen 
minutes. The key question, of course, was how much 
development was necessary to remove the mud without 
affecting the hydraulic properties of the aquifer materials 
outside the well? This was the main question studied in the 
field tests reported below. 

The series of testing performed on wells A, B. and C were 
identical with a few exceptions and consisted of four sets of 
tests (Table VI-1). The first set consisted of an ambient flow 
test and a pump-induced flow test which occurred before any 
well development. The second, third, and fourth sets were 
identical in procedure; air development, followed by an 
ambient flow test, followed by a pump-induced flow test. 
Some pump-induced flow tests were repeated following an 
overnight rest period (without a repeat of the air development) 


34 































































Table VI-1. Pump-Induced Flow and Ambient Flow Tests Performed on Wells A, B, and C 

Ambient Flow Tests Well A Well B 

Well C 

Pre-development 

— 

X 

X 

1st Development 

— 

X 

X 

2nd Development 

X 

X 

X 

3rd Development 

X 

X 

X 

Pump-Induced Flow Tests 

Pre-development 

X 

X 

— 

1st Development 

X 

X 

X 

2nd Development 

X 

X 

X 

3rd Development 

X 

X 

X 

Repeat Tests (Overnight) 

Ambient Flow Test 

1st Development 

— 

X 

— 

Pump-Induced Flow Tests 

1st Development 

— 

— 

X 

2nd Development 

X 

— 

— 


to determine if the results were repeatable. The ambient and 
pump-induced flow tests were conducted in the same manner 
as far as flowmeter position was concerned. The 
electromagnetic flowmeter was lowered to the bottom of the 
borehole where the first measurement (zero reference) was 
taken. Subsequent flow measurements were recorded as the 
flowmeter was raised at 1.52-m increments until reaching the 
top of the screen. The pump-induced flow was maintained at a 
constant rate and averaged about 34 L/min for all tests. 

VI-3 Test Results 
a. Shallow Well A 

Well A served as the standard for measurement procedures at 
the remaining wells. Due to the experimental function of this 
well, ambient testing was not performed until after the second 
development. Pumping tests were completed from the pre¬ 
developed stage through the three stages of development. Net 
flow and ambient flow plots (Figure VI-3), as well as 
differential net flow charts (Figure VI-4) of the flow 
distribution following the second and third developments, are 
presented to illustrate the effects of a third air-development. 


The ambient flow curve shown (Figure VI-3a) indicates that 
either the natural vertical flow has been altered somewhat by 
applying a third development, or the hydraulic conditions 
causing the ambient flow have changed. The natural flow has 
decreased by varying amounts at every measurement point 
except the uppermost point, and the greater shifts occur 
between depths of 15 m and 27 m. For example, at a depth of 
18.3 m the ambient flow after the second development was 3.6 
L/min, and at the same depth, after the third development it 
was 2.9 L/min, an eighteen percent drop. It is possible that 
variable hydraulic stresses on the aquifer resulted in the shifts 
rather than changes in well development. Several chemical 
processing plants and an electrical power plant, some of which 
place huge water withdrawal demands on the aquifer, are 
located within a 5-km radius of the testing site. 

The net flow plot (Figure VI-3b) shows the pump-induced 
flow data that have been corrected for the ambient flow 
measurements. Both data plots begin at 27.4 m with nearly 
the same nominal flow, and increase in tandem to about 15 m. 
At this point, the plots indicate that at least some variable 
development has occurred between depths of 6 m and 14 m. 
The 4.2 L/min flow variation at 9.1 m is a seventeen percent 
increase, but there is no clear evidence of its cause. This shift 


35 





Q. 

0) 

Q 


0.0 



1.0 


2.0 


3.0 


Ambient Flow (L/min) 

(a) 


4.0 


E 

sz 

*- 

a 

0 ) 

o 



Net Flow (L/min) 

(b) 


Figure VI-3. (Well A) (a) Ambient flow as measured by the EM flowmeter and (b) net flow (total flow - ambient flow). Data sets were 
obtained after the 2nd and 3rd developments. 


again may be attributable to external stresses placed on the 
aquifer. 

The differential net flow (DNF) logs (Figures VI-4) provide a 
picture of successive development effects. Each bar 
represents the amount of water released from the aquifer and 
entering a segment of the screen. Adding the flow of each 
segment should equal the net flow at the uppermost 
measurement station. The screened segment in this case is the 
length of the measurement interval. It is important to note that 
a comparison of DNF charts at different stages of development 
can be made only when the induced pumping rate Q p is 
common among the tests. 

Presumably, one would like to see a “shift and hold” pattern 
occurring after the first or second development. Shown in 
Figures VI-4a and 4b are the DNF measurements taken after 
the second development for two time periods. There is not a 
great deal of similarity in the two charts which indicates that 
the well needs further development to stabilize, or there is 
interference caused by stresses other than those produced by 
the pumping test. In comparing the results of the second 
development with the results obtained after the third develop¬ 
ment, there appears to be significant correlation. The upper 
and lower segments have relatively low DNF with the bulk of 
the DNF between 6.1 m and 21 m. 

b. Shallow Well B 

Well B is a shallow well having the same dimensions and 
construction as well A and is located approximately 10 m 


away. Each of the test series (pre-development through third 
development) was performed on this well for ambient and 
pump-induced flow. In addition, an ambient flow test after the 
first development was repeated after an overnight waiting 
period. The ambient flow data (Figure VI-5a) shows shifts in 
the flow profile after successive developments. In measure¬ 
ments obtained prior to development, the ambient flows are 
downward in the extreme lower and extreme upper sections of 
the screen, and upward elsewhere. After the first develop¬ 
ment, the downward flows disappear except for the extreme 
upward measurement point after the third development. 
Noteworthy at this point are the relatively high ambient flows 
in the mid-range of the screen when compared to the pump- 
induced flow rate of 34 L/min. For example, after the third 
development a measurement taken at a depth of 20 m 
recorded a 7 L/min ambient flow; this is about twenty percent 
of the pumping rate during the pump-induced flow test. The 
“shift and hold” pattern is quite evident in the measurements 
taken after the first and second development. Although a 
further increase in ambient flow is observed after the third 
development, this finding is not unusual considering the 
aquifer character and the sensitivity of ambient flow to 
external stresses. However, the cause of the large shift after 
the third development is not certain. 

The net flow curves (Figure VI-5b) indicate that successive 
developments had a measurable effect on the well. Each 
successive development showed slight decreases in net flows 
while maintaining the profile. The net flow curves correlate well 
with the ambient flow curves in that there were little changes in 
the upper and lower segments of the screen, and much larger 


36 







2nd Development 



Differential Net Flow (L/min) 


(a) 


2nd Development (overnight) 


E 


c 

o 

'*_* 

ro 

> 

a> 

LU 


-4.6 


-6.1 

1 

. - 


-9.1 

.:.. 1 

- 


-12 

1 

- 


-15 

J 



-18 

. . :.,i 



-21 

i 



-24 

.i 



-27 

. i 


1 1 


0 2 4 6 8 

Differential Net Flow (L/min) 


(b) 


3rd Development 



Differential Net Flow (L/min) 


(c) 


Figure VI-4. (Well A) Differential net flow (a) obtained after 2nd development, (b) obtained after 2nd development and an overnight 
waiting period, and (c) obtained after 3rd development. 


37 




























































































- 1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 

Ambient Flow (L/min) 

(a) 



Figure VI-5. (Well B) (a) Ambient flow as measured by the EM flowmeter and (b) net flow (total flow - ambient flow). Data sets were 
obtained prior to development and after 1st, 2nd, and 3rd developments. 


changes at the mid-screen level. It appears that for well B, 
ambient flow varied more than net flow, as seen between pre¬ 
development, first development, and third development. 

The largest differences in the distribution of DNF (Figure 
VI-6) occur between pre-development and first development, 
while only minor differences occur after subsequent 
developments. In addition, the large shift in ambient flow 
occurring after the third development is not obvious in the 
corresponding DNF chart. This result supports the conclusion 
that the ambient flow variations must be removed from the 
total flows to obtain consistent results. Since DNF reflects the 
flow entering a screen segment due to pumping, it is logical 
that pump-induced flow would be less responsive to 
development than ambient flow. 

For well B, an additional test was performed after the first 
development to determine the effects of an overnight time 
period on the ambient flows. No additional development was 
performed between these tests. The results (Figure VI-7) are 
quite similar to those obtained the previous day. 

c. Deep Well C 

The ambient flow in the confined aquifer screened by well C 
would be expected to be much smaller than those in the 
unconfined or semi-confined sediments above. It is known 
also that local industries placed less external demand on this 


aquifer than the shallow aquifers. The maximum ambient 
flow (Figure VI-8a) was about 1.75 L/min and was recorded 
prior to the first development. This flow is in contrast to 7 L/ 
min recorded as maximum ambient flow in shallow well B. 
Development of well C resulted in irregular shifts of the ambient 
flows, and the tendency was toward the aquifer having no natural 
vertical flows. This result is clearly seen in the measurement 
taken after the third development where the majority of ambient 
flows are less than approximately 0.5 L/min. 

Variability in the net flow curves following the first, second, 
and third developments is generally small throughout the 
range of measurements (Figure VI-8b). It appears that most 
development took place in the upper three measurement 
stations, which corresponds to the same area where the larger 
shifts in the ambient flow occur. 

The DNF profiles for well C are shown in Figure VI-9. 
Although the pumping test data before the first development 
were not available, the relevant sequential test data following 
that stage are presented. A moderate change in the profile is 
observed through the progression of developments. Some of 
the flow in the upper half of the confined aquifer has shifted to 
the bottom half, though most of the flow still remains in the 
upper portion. The DNF chart displaying the pump-induced 
flow test after an overnight period has the same general 
distribution as the first development pump-induced flow test 
performed the previous day. 


38 















CD 


ID 



(D0r-^NOCO(DO) 


-Jf.'T—T— t— C\JCMC\1C\] 

” n- i • i i « » • 

(ui) uo;;eA3|g 



(lU) U0!iBA3|3 




39 


Figure VI-6. (Well B) Differential net flow obtained (a) prior to development, (b) after 1st development, (c) after 2nd development, and (d) after 3rd development. 

























































































VI-4 Summary and Discussion 



(a) 


Figure VI-7. (Well B) Ambient flow obtained after the first 
development and repeated after an overnight 
period. 


As noted earlier, the key question is how much development is 
necessary to return disturbed porous media surrounding a well 
to a near-natural state. This study has presented some initial 
observations of the effects of successive developments on 
three wells. Two wells were shallow and screened in phreatic 
or semi-confined aquifers, and the third was a deeper, fully- 
penetrating well, screened through a confined aquifer. 

The results of tests on well A were inconclusive, in part 
because of a lack of ambient flow data from pre-development 
and first development stages. In addition to the lack of data, a 
good correlation between the flow distributions after second 
and third development did not exist as shown by the 
differential net flow charts. This was probably due, in part, to 
the experimental technique used at this well. There was, 
however, a similarity between a test repeated after the second 
development and the test performed after the third 
development. This similarity indicates that the first test may 
have been influenced by factors other than those imposed by 
the pump-induced flow. 

The differential net flow charts for well B show that the first 
development made a significant difference in the vertical 
distribution of flow to the well. Tests after the first, second, 
and third developments resulted in quite similar DNF profiles. 




Ambient Flow (L/min) 


(a) 


Net Flow (L/min) 


(b) 


Figure VI-8. (Well C) (a) Ambient flow as measured by the EM flowmeter and (b) net flow (total flow - ambient flow). Data sets are 

obtained prior to development and after the 1st, 2nd, and 3rd developments for the ambient flow and after the 1st, 2nd, and 
3rd developments for net flow. 


40 























41 


Figure VI-9. (Well C) Differential net flow obtained after (a) 1st development, (b) 1st development and an overnight period, (c) 2nd development, and (d) 3rd development. 



















































































However, as would be expected, the ambient flow data were 
less stable. Part of the irregularity may be attributable to the 
varying withdrawal demands placed on the aquifer by the local 
industry. 

The third well was screened in the confined aquifer, 
presumably more shielded from the hydraulic stresses beyond 
experimental control. The ambient flow in well C was small 
compared to the other wells, although measurable shifts were 
observed after each development. The irregularity of ambient 
flows may be due more to experimental “noise” than to any 
significant change in the natural flow. The differential net 
flow charts were moderately dissimilar after each 
development, indicating that slight adjustments were still 
being made to the media even after the third development. 

The results of the tests at the Mobile site were generally 
similar to those obtained at the Columbus site. Ambient flow 
profiles at both sites were more sensitive to effects such as the 
state of well development and minor changes in hydraulic 
stresses than induced flow profiles. In addition, the state of 
well development appeared to vary with differences in aquifer 
materials, as is expected. 


42 



Chapter VII 
Case Studies 


VII-1 Field Applications 

Issues related to aquifer heterogeneity are particularly 
important at sites where contaminant transport and fate will be 
characterized or subsurface remediation will be performed. A 
vertical-component borehole flowmeter may be used to 
investigate several aspects of ground-water flow relevant to 
monitoring and remedial design (Table VII-1). In order to 
demonstrate the utility of flowmeter investigations for gaining 
quantitative insight into aquifer characterization, results from 
several borehole flowmeter applications are described. 


Table VII-1. Characterization Objectives for Borehole Flowmeter 
Studies 


Measured 

Flow Log Phenomena That Can Be Investigated 


Ambient 

Conditions 


Pumping 

Conditions 


Direction of the vertical hydraulic gradient 

Cross-connection among geological units 
intersected by a well 

Active fracture locations/zones in bedrock 


Active fracture locations in bedrock 

Horizontal hydraulic conductivity distribution 

Identify zones of preferential flow and, 
potentially, contaminant transport for design of 
monitoring and remediation networks 


VII-2 Columbus AFB, Mississippi 

As discussed in Chapter V, borehole flowmeter tests were 
performed at two sites on the Columbus Air Force Base 
(CAFB). The two sites overlie a highly heterogeneous, 
unconsolidated and unconfined fluvial aquifer, are about 60 m 
apart, and are located approximately 6 km east of the 
Tombigbee River and 2.5 km south of the Buttahatchee River. 
Seasonal water table fluctuations range from 2 m to 3 m. The 
aquifer is composed of approximately 10 m of terrace deposits 
consisting primarily of poorly- to well-sorted, sandy gravel 
and gravelly sand that often occur in irregular lenses and 


layers. Visual inspection of the facies exposed at a quarry 
located a few kilometers from the site shows a complex series 
of lenses with significantly different physical and hydrological 
properties. The terrace aquifer is unconformably underlain by 
the Cretaceous-age Eutaw Formation, an aquitard consisting 
primarily of marine clay and silt. 

The Electric Power Research Institute performed the MADE 
study (Betson et al., 1985) at the first test site, which covers 
about 8 Ha. The MADE experiment included a network of 
258 multilevel sampling wells designed to monitor a 20- 
month, rapid injection, natural gradient tracer study. Boggs et 
al. (1992) provide a review of the tracer experiment. The 
dominant feature of the tracer plume is a highly asymmetric 
concentration distribution in the longitudinal direction. At the 
conclusion of the experiment, the more concentrated region of 
the plume remained within approximately 20 m of the 
injection point, while a more dilute portion extended 
downgradient a distance of more than 260 m. In certain 
respects, the plume configuration would appear to be the result 
of a continuous tracer injection rather than the rapid injection 
that actually occurred. 

One explanation for the skewed plume is that the tracer was 
slowly bleeding from a zone of relatively low hydraulic 
conductivity into relatively conductive zones, and thereafter 
moved rapidly downgradient. This explanation is supported 
by the hydraulic conductivity values from borehole flowmeter 
tests performed by Boggs et al. (1989) and Rehfeldt et al. 
(1989b). These particular flowmeter tests were performed 
primarily with an impeller flowmeter. The electromagnetic 
flowmeter was used only to supplement the basic data. Figure 
VII-1 shows a vertical cross section of the spatially correlated 
hydraulic conductivity profile along the longitudinal axis of 
the plume. The profile shows that the tracer was injected into 
a region of relatively low transmissivity and 25 m upgradient 
of a large region of relatively high transmissivity that 
contained several conductive lenses. Thus, the flowmeter 
measurements provide the basis for understanding the major 
characteristics of the tracer experiment. 

The first major application of the electromagnetic flowmeter 
in a granular aquifer was at the second CAFB site, which 
covers about 1 Ha. This is the location of the tests described 
in Chapter V. Numerous pumping, flowmeter, and 
recirculating tracer tests were conducted using thirty-seven 
fully-screened wells. Shown in Figure VII-2 are areal 
distributions for the depth-averaged hydraulic conductivity 
values for the uppermost and lowermost two meters of the 


43 





Figure VII-1. 


Figure VII-2. 


£ 


c 

o 

(0 

> 

o 

LU 


64 

62 

60 

58 

56 

54 

52 

50 

48 

46 



-0.2 < log K < 0.7 
-0.9 < log K < -0.2 
-1.6 < log K< -0.9 
-2.3 < log K< -1.6 
J_I_I_ 


-3.0 < log K < -2.3 
-3.7 < log K < -3.0 
-4.4 < log K < -3.7 
-5.1 < log K < -4.4 


FLOW 


-20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 

Distance (m) 


Vertical profile of hydraulic conductivity values along the longitudinal axis of the MADE tracer plume. 


£ 

>- 



-3.0 < log K < -2.3 
-3.7 < log K < -3.0 
-4.4 < log K < -3.7 


-5.1 < log K < -4.4 


20 40 60 80 100 

X (m) 

-0.2 < log K < 0.7 
-0.9 < log K < -0.2 
-1.6 < log K<-0.9 
-2.3 < log K < -1.6 


100 

80 

60 

40 

20 


Depth-averaged hydraulic conductivity values for the lowermost 2 m (left) and the uppermost 2 m (right) of the unconfined 
aquifer below the 1-Ha test site at Columbus AFB. 


44 

















saturated aquifer. In the upper portion of the aquifer, a band of 
high hydraulic conductivity crosses the test site. The location 
of this band matches the location of a former river meander 
shown in a 1956 aerial photograph of the site (Figure V-l). In 
the lower portion of the aquifer, the only non-clay material 
occurs in a depression of the lower aquitard near the center of 
the site. The S-shaped band of material with a relatively high 
hydraulic conductivity in the central portion of the figure is 
most likely sands and gravel deposited by an earlier river 
system. Thus, flowmeter data at the 1-Ha test site provided 
sufficient detail to delineate the sand and gravel beds of two 
former river channels. Being able to observe correlations 
between the hydraulic conductivity field and the geological/ 
depositional history of a study site provides the type of 
detailed data required to understand flow and transport in 
heterogeneous aquifers. 

VII-3 Oak Ridge National Laboratory, 
Tennessee 

The electromagnetic borehole flowmeter has been used to 
characterize the ground-water flow patterns in fractured 
bedrock at the Oak Ridge National Laboratory (ORNL) 

(Moore and Young, 1992; Young et al., 1991). The laboratory 
is located in the Valley and Ridge physiographic province of 
the Appalachian thrust belt of eastern Tennessee. The 
geological units consist of fractured sequences of calcareous 
shale, siltstone, shaley limestone, and limestone, which 
typically dip at angles between 45° to 60° from horizontal. 


Measurements at outcrops and on cores of the Conasauga 
Group show a density of 10-15 joints per meter in shale and 6- 
40 joints per meter in siltstone (Sledz and Huff, 1981). The 
joints and fractures are oriented parallel to the bedding planes, 
strikes, and dips of the lithologic units. 

At ORNL, the electromagnetic flowmeter has been used in 
open boreholes and in boreholes with sand-packed PVC 
screens. In most of the deep boreholes, the measured ambient 
flow profiles in the wells indicate that cross-communication 
was occurring between different fracture zones. Figure VII-3 
shows a profile of ambient flow where ground-water enters the 
well at a depth of about 100 m and exits at a depth of 41 m. 
The ambient flow is produced by natural upward hydraulic 
gradients, along with hydraulically active fractures at depths 
of 100 m and 41 m. 

At most of the wells, the permeable vertical zones are 
typically greater than 30 cm, which is the minimum vertical 
distance required for a steeply dipping fracture (>60°) to cross 
through a 17-cm OD borehole, the size of most of the 
boreholes at ORNL. Analysis of flowmeter logs indicate that 
the orthogonal spacing between fractures is about 0.15 m to 
0.44 m in the shallow bedrock, and about 0.44 m to 0.73 m in 
the deeper bedrock. Specific information regarding the 
location of fractures assists in correlating lithologic structure 
and hydraulically-significant fractures. Differences were 
observed in the flow profiles between different strata and 
regions. In some areas, a permeable flow zone is located near 
the top of bedrock. This zone was likely produced by 



0 

20 

40 

60 

80 

100 

120 

0.012 0.008 0.004 0 

Ambient Flow (L/s) 

(a) 



1_i_i 



Induced Flow (L/s) 


(b) 


Figure VII-3. (a) Ambient flow distribution in a well and (b) the flow distribution induced by constant-rate pumping. 


45 








weathering and serves as a pathway for significant shallow 
ground-water flow toward valley streams. 

At ORNL, newly drilled boreholes with depths near 500 m 
penetrate several fracture zones. In some cases, the different 
fracture zones contain ground waters of significantly different 
chemistry and from different recharge zones. Among the 
applications of the flowmeter data at ORNL is a refinement of 
the monitoring program based on the location of hydrauli¬ 
cally active fractures. Once the hydraulically active fractures 
are located, specific borehole zones may be isolated so that 
selective monitoring can include fluids from fractures or 
fracture zones and matrix fluids with long residence times. 
These data may be used to identify changes in water chemistry 
with depth and water chemistry signatures associated with a 
particular rock type. 

VII-4 The Oklahoma Refining Company 
Superfund Site, Oklahoma 

The Oklahoma Refining Company (ORC) Superfund Site is 
located in Caddo County on the eastern edge of Cyril, 
Oklahoma. Site topography consists of low, rolling hills with 
a deeply incised drainage system. Soils are characteristically 
red silty clay loams with low to very low permeability. 
Alluvium and terrace deposits are present in and around old 
stream channel sediments. Soils are underlain by the 
Weatherford Member of the Cloud Chief Formation (gypsum) 
in the northwestern part of the ORC site, and the Rush Springs 
Sandstone elsewhere. Ground water in the Weatherford 
commingles with that of the Rush Springs Sandstone, which is 
believed to act as an unconfined aquifer. The Rush Springs 
Sandstone consists of even-bedded to highly cross-bedded, 
very fine-grained, silty sandstone and outcrops on the eastern 
side of the ORC site (Bechtel, 1991). 

Electromagnetic flowmeter field demonstrations were 
conducted at three pairs of ORC wells. One well pair 
consisted of wells NE-2 and NE-3, which lies in a region 
where gypsum overlies sandstone. Figure VII-4 shows the 
flow distributions for ambient and induced-flow conditions at 
well NE-2. The flow distributions for well NE-3 were very 
similar. The profile of induced flow indicates that about 
eighty percent of the total flow to the well originates from the 
upper thirty percent of the well screen. This result suggests 
that materials in the upper portion of the screened interval are 
more permeable than those in the lower portion. 

The downward ambient flows indicate downward vertical 
hydraulic gradients and possible cross connection between 
different hydrostratigraphic units. At wells NE-2 and NE-3, 
about 0.2 L/min enters the well near the water table and 
migrates down the well, where it flows into the aquifer. Of the 
0.2 L/min, about half of it enters the sandstone at a depth 18 m 
below the top of well casing (TOC). If the recharging ground 
water was contaminated, ambient flows in the well would 
accelerate the downward migration of ground-water 
contamination. If the recharging ground water was clean but 
the deeper ground water was contaminated, ambient flows in 


the well could dilute ground-water contamination in the 
vicinity of well and in samples, hindering characterization of 
the ground-water contamination. 

In the southeast quadrant of the site, flowmeter tests were 
performed on a well pair consisting of wells IBB-4 and SBB- 
36. The ambient flows at these wells (Figures VII-4) are 
upward. Well IBB-4 was a flowing artesian well at the time of 
the test. The flow profiles at well IBB-4 indicate that the 
source of the artesian ground water is a narrow zone near a 
depth of 36 m. Boring logs indicate that a 0.8-m thick 
siltstone layer exists at the 36-m depth, which appears to be 
the source of the artesian ground-water flow. Well SBB-36 
shows two narrow zones of relatively high flow near depths of 
13.5 m and 15.5 m. The limited flowmeter testing at the ORC 
test site provided valuable information related to aquifer 
heterogeneity and potential contaminant transport. 

VII-5 Gilson Road Superfund Site, 

New Hampshire 

The Gilson Road site at Nashua, New Hampshire, was 
originally a 2.4-Ha landfill for refuse and demolition material. 
In 1979, it became a site for disposal of other wastes. Ground- 
water contamination formed a plume over 450 m wide and 33 
m deep, which was estimated to be moving at a rate of 0.6 ml 
day (Morrison, 1983). A bentonite slurry wall was constructed 
to contain the plume and the surface was capped with a 
synthetic membrane to prevent infiltration of rainfall. A 
ground-water pump-and-treat system was in operation for 
several years. 

The Gilson Road site is underlain primarily by stratified, 
unconsolidated sand and gravel deposits of glacial origin. The 
permeable sand and gravel deposits are underlain by a thin, 
discontinuous, low-permeability glacial till unit, that is up to 
3-m thick. Bedrock underlying the till is biotite schist of the 
Merrimack Group, which is differentially weathered and 
fractured across the site (Morrison, 1983). 

Electromagnetic flowmeter tests were conducted at four 
monitoring wells and three recovery wells of the pump-and- 
treat remediation system. The recovery wells were designed 
to fully penetrate the drift and till overburden. None of the 
available well logs indicated intersection of the upper bedrock. 
In all three recovery wells, the measured flow (Figure VII-5) 
was significantly greater near the top of the well screen. These 
results may be due to use of a coarse-grained filter pack in 
well construction and may not be representative of actual site 
conditions. However, if the results are representative of the 
shallow hydraulic structure in this aquifer, they would indicate 
potential inefficiencies in the recovery of ground water from 
the lower portions of the overburden using these wells. 

VII-6 Mirror Lake, New Hampshire 

The Mirror Lake drainage basin is located in the Hubbard 
Brook Experimental Forest in the White Mountains of central 
New Hampshire. The drainage basin is a small, well-defined 


46 



WELL NE-2 


WELL NE-2 



b) 





’ ?' 

9 

6 

I 

? 

I i 9 

i 6 

I 

" ? 



Pumping Rate = 3.8 L/min 

-I-1- 1 _l_I_I_L i_ I _ 

0 12 3 4 

Flow (L/min) 


WELL SBB-36 


WELL IBB-4 



Pumping Rate = 0.88 L/min 
Bottom of Screen 

-r...r—-- 


0.2 0.4 0.6 

Flow (L/min) 


0.2 0.4 0.6 

Flow (L/min) 


0.8 


—•— 

-o- - - 

Ambient 

Pumping 


Figure VII-4. Ambient and induced flow distributions for wells NE-2, SBB-36, and IBB-4 at the ORC Superfund Site. 


WELL I 


8 


10 

11 

i 

12 

i 

13 f 
( 

14 < 

:i5< 

< 

16 < 
< 

17 

18 


• Top of Screen 


%> 

■”“31 


pef' 


6 


1 


»9 

6 Bottom of Screen 


10 15 20 25 30 35 


Flow (L/min) 


WELL J 



Flow (L/min) 


-•- 

-o—- 

Ambient 

Pumping 


WELL K 



Figure VII-5. Ambient and induced flow distributions for wells I, J, and K at the Gilson Road site. 


47 




































hydrologic environment covering 85 Ha. A test site was 
established in the southwest corner of the basin by the U.S. 
Geological Survey for three primary purposes: long-term 
monitoring of bedrock environments; maintenance of a con¬ 
trolled field-scale laboratory to test new equipment, methods, 
and interpretive models; and characterization of fluid move¬ 
ment and solute transport in fractured rock (Winter, 1984). 

Bedrock underlying the Mirror Lake drainage basin is 
primarily composed of Silurian-age schist (Shapiro and Hsieh, 
1991). The schist is intruded by granite, and less commonly 
pegmatites and basalts. The rocks are extensively fractured 
from folding and faulting during successive orogenies. 
Overlying the bedrock is a layer of glacial drift, predominantly 
till, which varies in thickness from zero to 49 m. At the test 
site, bedrock is a granite dike, which is overlain by 12 m to 15 
m of glacial drift. 

Three existing wells, each drilled to a depth of 75 m, were 
chosen for demonstration of the electromagnetic borehole 
flowmeter. Well FSE-06 (Figure VII-6a) exhibited the largest 
ambient flow at 0.3 L/min. The results indicate two zones of 
significant transmissivity; an upper zone between 30 m and 35 m 
below TOC and a lower zone around 57 m to 64 m. The two 
zones are probably related to major rock fractures. Further 
evidence of transmissive fractures can be seen in the flowmeter 
profile obtained during pumping (Figure VII-6b). Ground water 
predominantly enters the well from these two intervals. 

Well FSE-09 exhibited much lower ambient flow (Figure 
VII-6c) than that detected in FSE-06, but the flows are high 
enough to indicate a hydraulically active fracture zone near a 
depth of 42 m to 46 m. The presence of this fracture zone is 
supported by the induced flow profile (Figure VII-6d). Very 
low magnitude ambient flows (Figure VII-6e) were detected in 
Well FSE-10. However, a large change in the ambient flow 
around a depth of 30 m to 38 m suggests that a transmissive 
fracture zone is present. Good correlation with the pumping 
induced flow data (Figure VII-6f) verifies the fracture zone. 
For comparison, results from acoustic televiewer surveys of 
wells FSE-09 and FSE-10 were made available by the U.S. 
Geological Survey. The televiewer images many linear 
features in the walls of both holes, including a relatively large 
feature at a depth of about 44 m in Well FSE-09. Similar 
general correlations between televiewer features and the flow 
profiles were observed in well FSE-10. 

Collective interpretation of the flowmeter logs indicates that 
there may be a network of hydraulically-active fractures at 
approximately 40 m below the ground surface in this area of 
the site. Data from these different logging methods are highly 
complimentary. The acoustic televiewer may be used to esti¬ 
mate the density and orientation of linear features, including 
fractures. The borehole flowmeter provides information to 
identify which features or zones are hydraulically active. 

VII-7 Logan Martin Dam, Alabama 

Logan Martin Dam is located in east-central Alabama in a 
complex geologic terrain. Situated on the lower Knox Group 


of the Pell City Thrust Sheet, the dam rests on a bed 
comprised of eighty percent to ninety percent dolomite which 
ranges from fine- to coarse-grained. The remaining rock is 
made up of a collection of cherts, shaley limestones, and fine¬ 
grained silica. The rock is highly fractured and creates a 
situation that is extremely conducive to diffuse ground-water 
flow. The hydrogeology is complicated further by the fact that 
certain layers of the underlying material are preferentially 
solutioned. This solutioning has created areas of conduit flow 
within the rock. 

Logan Martin Dam has experienced extensive problems with 
seepage and poor water quality downstream of the dam since 
its completion in 1964. The rate of leakage under the dam 
has increased from 168 m 3 /min in 1964 to approximately 
1104 m 3 /min, with the flow rate increasing only 84 m 3 /min 
since 1974. In 1968, it was observed that the flow rate at a 
monitoring weir increased sharply from 0.6 m 3 /min to 10.2 
m 3 /min. A steep-walled sinkhole then developed on the 
downstream side of the earthworks. Subsequent 
investigations revealed multiple sinkholes in the reservoir 
upstream of the dam. Grouting the earthworks and filling the 
sinkholes reduced the flow from the monitoring weir to the 
original 0.6 m 3 /min. 

Geologic mapping based on outcrops, lineaments, and core 
analyses suggested that a zone of rocks within the Knox 
Group, called Target Zone #1, was a major source of leakage 
(Redwine et al., 1990). The geophysical/hydraulic procedure 
to test this hypothesis was to use various existing wells and 
coreholes to run a suite of caliper, temperature, and flowmeter 
logs in an attempt to detect the vertical movement of water 
within the target zone, thereby documenting the qualitative 
information resulting from the earlier geologic study. Of the 
wells that were tested, most were clogged with mud at various 
depths, thus no substantive data were obtained from these 
tests. However, two of the wells (Well 332 and Well 301) 
were clear for logging. 

At Well 332, upward flows of about 130 L/min were 
encountered between 78 m and 110 m above mean sea level 
(AMSL) (Figure VII-7). The natural flow entered the 
borehole at 78 m to 80 m AMSL and flowed up the borehole 
until reaching an exit point at about 110 m to 112 m AMSL. 
These data were interpreted as indicative of large fractures or 
conduits at these elevations. 

Well 301 provided different results. At 1.5 m AMSL, a flow 
of 51 L/min was measured entering the borehole. This flow 
steadily decreased to 43.3 L/min at 29.5 m AMSL where the 
flow then disappeared completely over a short section of the 
borehole. The flow then resumed at 30.5 m AMSL and rose to 
a maximum at about 33.5 m AMSL, then decreased, first 
slowly and then rapidly, until 48 m AMSL, where flow was no 
longer detected. The decrease of flow occurring between 29.5 
m and 30.5 m AMSL was due to a large solution opening 
where the cross-sectional area for flow was enlarged 
dramatically, leading to a substantial decrease in flow velocity 
in the wellbore. This expansion in the wellbore was shown on 
a caliper log. 


48 



10 

20 

30 

40 

50 

60 

70 

80 

-i 

10 

20 

30 

40 

50 

60 

70 

80 

-i 

10 

20 

30 

40 

50 

60 

70 

80 

-( 

- 6 . 


a) Well FSE-06 

. BpilPIU of_Casin 2 __ 



Bottom of Borehole 


e) Well FSE-10 


02 -0.015 


-0.01 -0.005 

Flow (L/min) 


; 


3 -0.25 -0.2 -0.15 -0.1 -0.05 0 




b) Well FSE-06 

_ _Bottom_of_C a s i_n2_ 2 


< i 


o- - 

o 

d 


9 

9 

-°-o 


Bottom of Borehole 










0 

2 

4 

6 

8 

10 

12 

14 


d) Well FSE-09 

^BottomjofJDasin^ 


" -TT 


er 

/ 

/ 

9 

\ 

-6 


o — 

\ 


i 

* > p 
-* a 




Bottom of Borehole 



0 2 4 

6 

8 

f) Well FSE-10 

Bottom of Casing 


...JL _ 


_ - -© 


o - 

_ -6 


f9- 

\ 

9 

i 


_ - e 


Bottom of Borehole 


0.005 0 


0.5 


1 1.5 2 

Flow (L/min) 


2.5 


-•- 

— -o- 

Ambient 

Pumping 


imbient and induced flow distributions for wells 


FSE-06, FSE-09, and FSE-10 at Mirror Lake, New Hampshire. 


49 








































Well 332 


Well 301 



c 

o 

03 

> 

Q 

LU 


Figure VII-7. 




Flow (L/min) Flow (LVmin) 


Substantial ambient flow moving from one stratum to another as detected by an impeller flowmeter. The flow is moving 
under a dam, the base of which is at an elevation of about 140 m AMSL. Flowmeter data were used to help select a 
geologic model for explaining the large leakage of water low in dissolved oxygen that was observed below the dam. 


All detected flows were upward, indicating that water leakage 
was from around and/or under the reservoir. It was concluded 
that the borehole flowmeter was a valuable device for locating 
transmissive features and directly measuring flows of ground 
water in fractured rock systems. 

VII-8 Cape Cod, Massachusetts 

In a study by Hess et al. (1992), a comparison of the 
variability of hydraulic conductivity was presented for in situ 
borehole flowmeter measurements and lab permeameter 
measurements. The theory and application pertaining to the 
flowmeter data are those techniques given by Hufschmied 
(1983, 1986) and Rehfeldt et al. (1989a, b). Much of what 
follows comes directly from Hess et al. (1992). 

Field studies into the characterization of aquifer heterogeneity 
were conducted in an abandoned gravel pit south of Otis Air 
Force Base at Cape Cod, Massachusetts. The unconfined 
aquifer is located in a glacial outwash plain and is composed 
of clean, medium-to-coarse sand and gravel, containing 
typically less than one percent silt and clay. The aquifer is 
about 30 m thick and is underlain by less permeable silty sand 
and till deposits. The water table is about 14 m AMSL and 
about 6 m below land surface. The hydraulic gradient is 
approximately 0.0015 m/m and indicates a southward flow 
direction. Ground-water seepage velocity averages 0.42 m/d 
as calculated from average properties of the aquifer (LeBlanc 
et al., 1991). Surface exposures of the outwash reveal 
interbedded sand and gravel deposits. Crossbedded troughs up 
to 1 m wide, but typically less than 0.5 m high, are observed in 


exposures perpendicular to the hypothesized paleocurrent 
direction, which is north to south. Parallel to this direction, 
troughs exhibit a tubular form and are several meters long 
(Hess and Wolf, 1991). 

Sixteen wells were installed for flowmeter tests. Fifteen of the 
wells were installed by a drive-and-wash technique to 
minimize the disturbance of the aquifer region surrounding the 
well. One well was installed using a hollow-stem auger so 
that core material could be obtained for permeameter analysis, 
which could then be directly compared to the flowmeter 
analysis. In addition, core samples were taken from the upper 
6 m of each of the boreholes. Each 5.1-cm diameter 
flowmeter well was screened over the upper 12 m of the 
saturated zone. Only the top 6 m to 7 m of the profiles were 
used for comparisons. These intervals correspond to the core 
sample zones, and also to the horizon of the aquifer through 
which the tracers moved during an earlier tracer test. 

Multiport, constant-head permeameter analysis of the core 
samples was the second method used in this study to obtain a 
measure of hydraulic conductivity. Measurements were taken 
from each section of the core between adjacent manometer 
probes inserted at 5-cm to 10-cm intervals along the core 
sample length. Summary statistics, mean, variance, and range 
for the hydraulic conductivity data sets from the two methods 
are presented in Table VII-2. The flowmeter data have 
significantly higher mean and variance values than do the 
permeameter data. The flowmeter geometric mean of 
0.11 cm/s lies within the range of values previously estimated 
for this aquifer and is only slightly lower than the value 


50 













Table VII-2. Summary Statistics of Hydraulic Conductivity Data 
Obtained Using a Borehole Flowmeter Method and 
a Permeameter Analysis Method (from Hess et al. 
(1992)) 


Flowmeter Permeameter 


Number of wells or coring locations 

16 

16 

Number of hydraulic conductivity values (K) 

668 

825 

Mean vertical spacing (cm) 

15 

7.3 

Geometric mean of K (cm/s) 

0.11 

0.035 

Variance of In K (a, 2 ) 

0.24 

0.14 

Range 



Minimum K (cm/s) 

0.013 

0.006 

Maximum K (cm/s) 

0.37 

0.14 

Range in horizontal spacing (m) 

0.9-24 

0.9-24 


estimated from the aquifer and tracer tests (0.13 cm/s). The 
permeameter mean of 0.035 cm/s is significantly lower than 
previously reported values. Several potential reasons for the 
low permeameter calculations include: local-scale anisotropy, 
nonrepresentative sampling techniques, and the influence of 
compaction during the coring process. In comparison, the 
flowmeter appears to supply more representative data, and is 
an easier and cost-effective alternative to core sampling and 
permeameter measurements. 

VII-9 Summary 

Sensitive borehole flowmeters, including the electromagnetic 
flowmeter described in this report, are valuable tools for 
detailed characterization of hydrogeology. In many cases, 
these tools may be used in existing wells to obtain such 
information as natural ground-water flow patterns within the 
well, identification of hydraulically active fracture zones in 
rock, and, potentially, estimates of the relative hydraulic 
conductivities of materials adjacent to the well screen. In 
conjunction with other data, including information from 
geologic and geophysical logs, these results may be used to 
define the hydrostratigraphy of aquifer formations and 
evaluate such issues as contaminant transport and fate, 
efficient design of extraction and injection systems for 
subsurface remediation, and monitoring network design. 


51 
































' 












































































































Chapter VIII 
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53 



Hess, A. E., and F. L. Paillet, 1990. Applications of the 

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55 



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56 



Appendix A 

Field Data Sheets for Borehole Flowmeter Tests 


TVA ENGINEERING LABORATORY 
EM FLOWMETER FIELD DATA SHEET 


Project Name / Project Abbr. (xxx) 


QA Number (xxx....) 


Date 

Start Time 

End Time 





Survey By 


Pre-Calibration Ref No. 


Flowmeter Cal 

Flowmeter Zero 

LPM / Volt 

Volts 


Gain Switch 

Integration Switch 

Excitation Voltage 

1 | HI □ L0W 

1 1 Is n 10s □ 20s 

VRMS 


Packer 

Collar (Serial #) 

1 | Yes | 1 No 

1 | Yes □ No 



Flowmeter Serial # 


Electronic Serial # 


Cable Serial # 


1 1 Pump 1 1 Injection 1 I Ambient 


Rate 



Pump Intake Depth 






GW Temperature 


Well Number: 


Depth to Water: 


Top of Reference Elevation: 

Construction Type:_ 

Pipe Size: _ 


Depth to Top of Screen: 


Depth of Bottom of Screen: 
Depth to Bottom of Well: — 


Screen Type: 


Blank Sections in Screened Interval: 


Comments: 


57 





























































Appendix A 

Field Data Sheets for Borehole Flowmeter Tests (continued) 


TVA ENGINEERING LABORATORY 
SINGLE-WELL FIELD DATA SHEET 


Project Name / Project Abbr. (xxx) 


QA Number (xxx....) 


Well Number 

Survey By 






Date 


Flow Check 


Time 

Volume 

Time to Fill 

Flow Rate 


































































Depth of Transducer: 


Initial Transducer Reading: 


Datalogger File Name: 


Pumping Schedule 


Time (hr, min) 


Rate (LPM) 


1 . 

2 . 

3. 


Drawdown Check 


Pump Type: 


Pump Serial # 


Transducer Type: 


Transducer Serial # 









Time 

Electric Tape 
Depth 

Datal 

Reading 

ogger 

Depth 






















Comments: 





RBHRBHHRRHS9B9S9BRS9BRHRRHBMERB9HHHMHH8HMBMHHHHBRBRHNHHRMHR 


58 



















































































Appendix B 
Equipment Checklist 


Address & phone number of contact 
. Site maps 
. Well specifications 
. Calibration sheets 
. Test forms 
. Equipment manuals 
. 1/2-inch flowmeter 
. 1-inch flowmeter 
. Flowmeter electronic system 
. Computer interface (6B12, HPIB) 

. Flowmeter spare parts 
. Flowmeter collar 
. Collar weight 
. Packer assembly 
. Packer inflator 
. Air compressor 
.Air tank 

. Pressure inflator tank 
. Inflation tubing, fittings, and valves 
. Flowmeter cables 
. 5 psi pressure transducer 
. 10 psi pressure transducer 
. 20 psi pressure transducer 
. 30 psi pressure transducer 
. Flowmeter computer 
. Electronic data logger 
. Printer 

Computer, printer, transducer, & electronics cables 
. Cable for downloading data logger 
. Floppy disks 

.Application and operating software 
. Flowmeter software 
. Datalogger software 
. Other software 
. Peristaltic pump 
. Submersible pump 
. Centrifugal pump 
. Other pumps 
. Valves 
. Inlet hoses 
. Discharge hoses 
. Checkvalve 
Plose clamps and fittings 
Pump calibration containers 
Flowmeter calibration cylinder 
Water containers 


_OSFIA Training Card 

_Stop Watch 

_120 ACV generator 

_220 ACV generator 

_50 ft 120 ACV extension cord 

_50 ft 220 ACV extension cord 

_25 ft 120 ACV extension cord 

_Special extension cords 

_Ground fault box 

_Power strip 

_Gas can 

_Weight for steel tape 

_Engineering rule 

_Steel tape 

_Tools (electrical and mechanical) 

_Flack saw 

_Drill and bits 

_Tie wraps 

_Electrical tape 

_Flashlight 

_Electrical supplies (splicing kit, solder, wire, etc.) 

_Misc hardware (screws, bolts, etc.) 

_Marking tape 

_Indelible pen 

_Rope 

_Tarpaulins 

_ Canopy 

_Tent 

_Spring clamps 

_Parachute cord 

_Multimeter 

_Oscilloscope 

_Rain suit 

_Boots 

_Cold weather gear 

_Sun screen 

_Insect repellent 

_Cooler and drinking water 

_Stools, chairs 

_Portable table 

_Cart 

_Decontamination supplies (buckets, water, soap, brushes) 

_Rubber gloves 

_Work gloves 

_Rags, paper towels 

First aid kit 




59 


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